It's very informal, American English to mean relaxing in some way. "Kick back" refers to putting your feet up on a desk, sitting down, and not using any energy. "Yo, let's kick back, relax, and have a few beers" --It is commonly used among friends, family, and close colleagues.

Some guys use it, like frat boys and grown up frat boys. And those speaking with a more hip-hop vernacular. A 1990's one. Actually, those are the same group, mostly. Come on bro, kick back and have a few brewskies. Ugh.