This directory is used to allow different products to customize settings for repos which are DEPS'ed in or shared.
For example: V8 can be built on its own (in a "standalone" configuration), and it can be built as part of Chromium. V8 defines a top-level target, //v8:d8 (a simple executable), which will only be built in the standalone configuration. To indiate itis a standalone configuration, v8 can create a file, build_overrides/v8.gni, containing a variable, build_standalone_d8 = true and import it (as import("//build_overrides/v8.gni") from its top-level BUILD.gn file.
Chromium, on the other hand, does not need to build d8, and so it would create its own build_overrides/v8.gni file, and in it set build_standalone_d8 = false.
The two files should define the same set of variables, but the values may vary as appropriate to suit the the needs of the two different builds.
The build.gni file provides a way for projects to override defaults for variables used in //build itself (which we want to be shareable between projects).
TODO(crbug.com/588513): Ideally //build_overrides and, in particular, //build_overrides/build.gni will go away completely in favor of some mechanism that can re-use other required files like //.gn, so that we don't have to keep requiring projects to create a bunch of different files to use GN.