Chromium's base library. Includes basic utilities useful across the codebase, e.g. hash maps, containers, pointer wrappers, logging, metaprogramming utilities, etc.

Inside Google there's a C++ library of many common utilities called "base". The base in Chromium is an ancient fork of it that's evolved within the Chromium codebase over many years. Abseil is another descendant of the Google internal base library.

WTF (Web Template Framework) is the similar library from WebKit, that also exists in Blink, though the two are quite different these days.

Chromium uses a build tool called bazel. //base is a bazel path, where // is the root of a workspace (which is usually a version controlled repository). Thus it is referring to https://chromium.googlesource.com/chromium/src/+/refs/heads/...