When we talk about lumens, we are talking about the level of energy that is being released by a light source in the form of electromagnetic waves and photons of energy. This energy is the key to all life on Earth(hows that for a statement :hubba
But its true, the plants use this energy to power the chemical reactions that go on in their leaves, which in turn are used to create all of the complex compounds used in the plant to grow it vegetatively and more during flowering.
MJ is a high energy plant and requires a specific minimum amount of light energy (measured in lumens) in order to thrive. Notice I say thrive not survive. Where a lot of people go wrong with growing MJ is that they think of MJ as a "houseplant". It is anything but a "houseplant". These people give the plants just enough energy to survive but not thrive, which is whats needed to produce the quality yields you see peeps here posting.
That said, we measure lighting needs not by number of plants but by the size of the space the plants are in(square footage of lit space). With that in mind it is completely concievable to have a 1000w HPS light over a single plant and still be underlit. The reason is the Physics behind light energy dispersal. The more area a given amount of light has to cover the less amount of light enerrgy there is contacting any given spot. This loss of light energy is also affected by vertical distance as well. There is actually an algebraic formula for figuring the light dispersal within a cubic space since light energy dimenishes over distance. But don't worry too much over that, as we have a relative standard that takes that into account.
Basically you want good quality "blue/white light" for vegging plants. This is given in spectrum as 5000Kelvin, 6000Kelvin, 6500Kelvin. The higher the Kelvin number the bluer to UV the light spectrum goes. Many of us here have found the 6500K lighting to be about the best spectrum of light for vegging plants and maintaining nice tight growth. The amount of light given off by the source of a CFL, T5 flourescent tube, Metal halide, or High Pressure Sodium light is given typically in the US in Lumens. The amount needed for thriving growth is a minimum of 3000 lumens per square foot of lighted area. Any of these lights will have the ability to give this level of light (although not in the same spectrum which is also important).
Since we all are trying to do this for the least amount of cost, we try to get the most cost-effective setups to match our needs. The CFL is the least efficient of the lights I listed (I don't list LEDs because they are a different animal). The T5 high output floro-tube is about as efficient as the Metal halide(maybe more in some ways). The the most powerful yet is the HPS, and at this time, the 600wHPS is the most efficient as it puts out the most lumens per watt of electricity used.
Many people like to use the CFLs because they are compact(and cheap) and work in smaller spaces where less light is needed. Often people will use multiples of these in the cloning and seedling stages where less than 3000Lpsf is acceptable. But once the plants start growing in earnest, the 3000L minimum is needed, with 4-5000Lpsf being better (the ungarded sun is about 10,000Lpsf on average). Once the plants reach the point that they can be induced into flowering, the light energy needs to be increased to accomodate the loss of 12hrs of light per day as you are taking the plants from 24hrs of light down to half at the flick of a switch. Nature does this very gradually and allows for the plants to acclimate. Because we are wanting to make it happen quicker, we cause a great amount of stress on the plant which makes it think that it has become hidden by some external barrier, and begins to stretch itself out to try to find the light again. By increasing the level of lumens to a minimum of 5000Lpsf (with 6-7500 being better), we can offset the shock of loss of light and minimise the stretching as much as possible while still giving the plant all the energy it needs to build the flowers and chemicals we so enjoy. At the point of flowering, most of us switch over to the HPS lighting as that is the most efficient of the light sources, and it gives an added bonus of having the "spectrum" of light that the plants like to have for building flowers (yellow/amberish 2700-3200Kelvin).
If you look up any of these lights, especially in the hydro/grow stores, they will list the spectrum and level of lumen output for a given light fixture or wattage. For example, the 600w HPS gives off about 90-95,000L per light bulb. So for flowering with under the 600w, you could properly light a sqft space of 16sqft, or a 4'x4' area. that same space used for vegging would only need 50-60,000 lumens of light. A 600w metal halide or a 4'T5 fixture with 10 bulbs would work for vegging in that space.
Sorry for the "Lecture on light", I love the science of it all. I hope this help you understand it a little better.