Tailored Access Operations. It's the "offense" branch of the NSA, responsible for gaining access to external computer systems: often technologically, sometimes legislatively when they're domestic. They did QUANTUM and FOXACID among other access tools.

It's a major part of the NSA, and generally considered to be where the bulk of the "serious hackers" work. The Equation Group is (probably) tied to TAO - they're the access group that was recently affected by the Shadow Brokers leak.