a crate may contain may different parts, and you may not always want all of them to avoid bloat. For an example, a crate may contain a sync and an async version, but you will probably only want one of them. So then the crate exposes the different parts as features. In clap they have defined the default features as:

default = ["std", "color", "help", "usage", "error-context", "suggestions"]

So, then if you need to use functionality not included there, then you have to specify it in your features list. On https://docs.rs/crate/clap/latest you can find a drop down in the top menu with Feature Flags, then they also have the documentation for the feature flags here https://docs.rs/clap/latest/clap/_features/index.html