"Irasshaimase!" the chefs are all yelling in unison the moment you enter their restaurant.
It's a surprise the first time it happens but get used to it, it's standard practice throughout Japan. It means basically "welcome to our establishment", since it's used in shops as well, but it flourishes in restaurants and bars.
The greeting is another example of the rarefied and excessive norms of politeness that are a Japanese specialty. No response is required, particularly from a foreigner, but a nod of the head is a nice acknowledgment, and you can even say "domo", which is a "thank you" catch-all.
If you want to be super polite you can say "ojama shimasu", which means "I'm going to disturb you", and to the Japanese way of thinking that's an apology.
With overuse, "irasshaimase!" has lost some of its currency and become a little less than heartfelt in some places. Especially in convenience stores where it's often muttered without much conviction, like the standard "have a nice day" that staff at the checkout counter in the US often mutter as you're about to leave.