Max Degrees of Parallelism (MAXDOP) tells the database engine, “If you decide to parallelize a query, go parallel with this many worker threads.”
(It’s a little more complex than that – there is also a coordinating thread, plus a single plan might have multiple parallel zones that each consume MAXDOP worker threads, but for the sake of this blog post, let’s keep it simple.)
Microsoft’s recommendations on how to set MAXDOP are a little long-winded, but again, in the interest of brevity, I’m going to summarize it as setting it to the number of cores in each physical processor, up