When you enter a store, especially a smaller one like a convini, one of the clerks will shout at you. It’s not because you’re a dirty foriegner and they are telling you to leave. This shouting is actually a good thing.
The clerk is shouting “Irashaimase.” It’s a very polite form verb, and as such can sometimes be difficult to find in a pocket dictionary. Assuming you can understand the pronunciation, as it is often said quickly. It is being used here as a greeting, like saying “Welcome to our store.”
You don’t need to respond, but if you want you can reply “domo.” At least now you won’t have to stare back blankly thinking “I have no idea what you just said to me.” Like I did.