It is a repo with very early C compilers and other code written by Dennis Ritchie. I found it on a Ritchie's home page in Bell Labs.
Currently, GitHub saves their data in The GitHub Arctic Code Vault and I think that the historical code must be saved for our descendants and in memory of Dennis Ritchie.
You can't build this code but you can read it. And you can use various tools to try build and use it. For this purposes please see The Unix Heritage Wiki.
Due to Dennis Ritchie our world and our computers are as we known them.