These aren’t something that I ever want to see and if I do I am straight away going to go looking for the cause. Basically what’s that’s telling you is that you’ve got processes waiting on a memory allocation. This is going to be because SQL Server hasn’t got enough memory to dish out to that particular process.
What’s going on here is that every time a query runs, it’ll ask for a certain amount of memory. A happy SQL Server will serve up a tasty slice of memory for that query to run in. The problem comes when the query is asking for a bigger slice of the pie than SQL has available. In that case the query will need to wait until SQL has enough free to give the query what it wants, that’s when the RESOURCE_SEMAPHORE wait starts ticking up.
Read on for a second look at this, as well as what you can do to help. Sometimes the answer is “add more memory” but this isn’t necessarily the case.