"If you snooze you lose" is slang in American English. I am not sure if it is also used in British English. It means if you take too long to take advantage of an opportunity or make a decision, you will miss out on something.. You can use it with friends or with colleagues in very informal situations. For instance, if you are watching something on eBay, and you cannot decide if you want to bid, so you wait. Unfortunately, when you go back to bid, someone has purchased the item with "Buy it Now". In this case, "You snooze you lose"

The word "snooze" is a casual way of saying "sleep". The expression means that if you are asleep (not paying attention) when you should be awake and alert, you are going to be unsuccessful in some way, that is, you are going to lose. You could use this expression to give a friendly warning to someone who may not be paying attention or alert to an important event. It sounds a bit silly and old fashioned to say, and for that reason it mutes the severity of the warning.

sentence** It means if you don't act now, you may miss an opportunity. Snooze means sleep, so it implies if you are lazy or tardy, you will not get everything you could get if you applied yourself.