What is the difference between Readable,Visible and Read Only in permission set?

Read-Only: If you check the “Read-Only” checkbox, that field will be read-only for the users of that profile. The user can only see that field, he can’t edit. The field must be Visible to be Read-Only.

Visible: If you check the “Visible” checkbox, that field will be visible to that profile. User can read & edit that field

Readable: Read permission on an object gives access to view records that are either created by users themselves or any record that is being shared by sharing rules/role/manual sharing