Topping is when you cut off the top part of your plant.
Generaly speakng, it doesn't increase yield (unless you increase nutrients, lights and time of veg). It means twice as many buds, but 1/2 size vs. untopped.
When you top a plant properly, two branches grow from that point, and those two branches grow to the sides.
People top their plants for several reasons.
1. To restrict vertical growth, like if youre limited in the hight of your indoor growroom.
2. Outdoors, to keep them from being too noticeable, since untopped plants can grow to 7' tall, even 12' tall in cases.
3. Outdoors, to prevent the tell-tale x-mas tree shape that pigchoppers look for.
4. Outdoors, with multiple toppings, you can make the plant grow in a hedge or shrub shape. Then you can tie plastic flower/berries to the branches, for stealth.
-To top, simply make a straight-across cut, midway between nodes. It's best to do this with a clean razor blade.
-You can clone the top you cut off.
High 420, to top a plant is when you remove a section of the main stem, usaly the very tippytop above an established node (braches). when new tops form (you'll have 2) you can do this again and end up with 4. It's useful for keeping a shorter bushier plant if space is a problem. But if space isnt a problem just plant more and dont top. topping drags the harvest on a bit but it depends if your in a hurry or not. theres quantity issues too. 4 plants or two topped plants? 4 plants un topped would be faster and bout the same amount. you will probably get more info from some of these grow sites