RESOURCE_SEMAPHORE_QUERY_COMPILE – this means a query came in, and SQL Server didn’t have an execution plan cached for it. In order to build an execution plan, SQL Server needs a little memory – not a lot, just a little – but that memory wasn’t available. SQL Server had to wait for memory to become available before it could even build an execution plan. For more details and a reproduction script, check out my Bad Idea Jeans: Dynamically Generating Ugly Queries post. In this scenario, cached query plans (and small ones) may be able to proceed just fine (depending on how much pressure the server is under), but the ugly ones will feel frozen.
Read on for a couple more examples of poison wait types.