what does it mean when you turn on your personal hot spot, when it says Allow others to Join

It means exactly that. It turns on the feature to permit others to attempt to connect to your personal hotspot.

Personal Hotspot can be used with your own devices, or can be shared with other people's devices.

It also explains it it in gray text a bit further down:

They will still need the password set up underneath the switch to connect however.

This message means that other people can see your Hot Spot and join it if they have the password.