I don't understand breeders

The thread title sounds like a complaint you might hear on certain other Internet message boards.

Do you understand nuclear reactors and how they work? Have you read the wiki page? Which part in particular are you having trouble grasping the concept of?

Looking at your post's signature, if you have trouble reading English, you may find it more productive to Request a Translation of the key facts in to your native language; or possibly even use an automated translation tool such as translate.google.com to get a rough idea of what the rules are (rough enough to begin experimenting).