to chill = to hang out, usually with your friend(s). If you're chilling, you're probably just doing nothing, or you have nothing specific planned, thus the expression 'just chillin(g)' ///// Time to chill = it's time to calm down. If someone tells you to 'chill' or 'you gotta chill' (the older expression is 'chill out') it usually means you're acting crazy or you're worrying too much.

"Chill" is just a slang expression for 'relax.' So if you're chilling, you're relaxing. (In formal language to chill is to make something cold, for instance by putting it in the fridge.)

its slang for time to relax

If you are asking about the difference between this two phrases, it's the same as: It's time for dinner. It's dinner time. It's time to chill. It's chill time. The first is the full expression ('It's time for us to chill' - the same as 'It's time for us to eat') , and the second uses a two-word noun phrase 'chill time'. The meaning is the same.