DMVs

Andy Yun talks about his favorite SQL Server feature:

Much earlier in my SQL Server career, I never knew much about how SQL Server operated under the hood. I only had knowledge of the things I could do directly, but I never questioned what was really happening behind the scenes. Then I attended SQLskills IE1 training, which was a turning point in my career. Among other things, it was my first exposure & deep dive into SQL Server Internals. I became enamored with learning how things really worked under the hood and the DMVs/DMFs became one of my best friends.

This is a big part of SQL Server administration.  It boils down to asking the server where it hurts and understanding how it responds.  DMVs are extremely helpful in that regard.