This folder has traditionally held the source of Python based tools used by EDK2. The official repo this source has moved to https://github.com/tianocore/edk2-basetools. This folder will remain in the tree until the next stable release (expected 202102). There is a new folder under Basetools BinPipWrappers that uses the pip module rather than this tree for Basetools. By adding the scope pipbuild-win or pipbuild-unix (depending on your host system), the SDE will use the BinPipWrappers instead of the regular BinWrappers.
The discussion is on the mailing list. The RFC is here: https://edk2.groups.io/g/rfc/topic/74009714#270 The benefits allow for the Basetools project to be used separately from EDK2 itself as well as offering it in a globally accessible manner. This makes it much easier to build a module using Basetools. Separating the Basetools into their own repo allows for easier CI and contribution process. Additional pros, cons, and process can be found on the mailing list.
By default, EDK2 is tied to and tested with a specific version of the Basetools through pip-requirements.txt. You can simply run:
pip install -r pip-requirements.txt
This will install the required module, thought we strongly suggest setting up a virtual environment. Additionally, you can also install a local clone of the Basetools as well as a specific git commit.