The “Tome” is a collection of ideas for additions and homebrew content for D&D. Typically it is targeted for 5e, but I’m sure other editions and other TTRPGs could utilize the content with minor changes.
There are is a collection of “rough ideas”, more fleshed out “complete” features, and even a rare few “tested” ideas that will have proven to complete enough that I have used it in a session in some way.