It's an optional tag which can be used "to specify the type of passenger public transport vehicle that uses a stop or station". highway=bus_stop does infer that it's for buses but I think it's better to explicitly state that as well.

https://wiki.openstreetmap.org/wiki/Tag:bus%3Dyes#:~:text=to%20specify%20the%20type%20of%20passenger%20public%20transport%20vehicle%20that%20uses%20a%20stop%20or%20station