I have a chicken proof lawn.

I have a chicken proof lawn. Its impeccable.

This joke is a pun. It is based on the fact that the word impeccable sounds like "in-peck-able", which apart from being not a real word, has the obvious meaning of "unable to be pecked". Chickens (I know this from personal experience) really love picking up lawns, so that would be a pretty good attribute to have on a chicken proof lawn.