Ok, a phenotype is talking about genetics. Just like humans you can have kids from the same parents but they will all come out a little different. Thats like phenotypes. You will have the same strain, but that same strain will produce different variations, some more desirable then others. Thats what breeders do. Breed a lot of a specific plants (plans meaning a mother and father plant) then they take the best phenotypes they find from the batch of new plants made from the mom being pollinated from the father.
A good example is say a couple has 2 kids. One kid is a major league baseball player and the other is a couch potatoe. If your creating good genetics your going to want to pick the best one. Same with breeding plants. You want the major league baseball player of your phenotypes.
Does that make sense to you? Hopefully I explained it in a way you could understand.
My blog hasn't gotten this far into it yet, its still new, but eventually I will be talking about this stuff. So if you like you can check out my blog and keep up with it. I will have lots of usefull info soon along with marijuana news recipes and other shit.
http://Cannabis-Country.blogspot.com