Unable To Map Users To The Login

I am new to this, so, I will be greatful to anyone who can clarify the following:

qn 1:
In login properties under Logins of Security, what does creating mapping to the different databases mean ?

For a given login, I notice that I can login / execute DDL statements alike for the databases which are checked and also for the ones that are not checked.

Then, what does
Security -> Logins - > Login Properties -> User mapping -> Users mapped to this login -> checking the database - accomplish?

qn 2:
For a perticular login, say 'Bob' which has server admin rights, I wish to include two more databases. For this I 'checked' the databases, which I wish to include, in the


Security -> Logins - > Login Properties -> User mapping -> Users mapped to this login

When I clicked 'OK', I get the following error and the two databases are left unchecked.

User, group, or role 'SQL1AGENT' already exists in the current database. (.Net SqlClient Data Provider)

How can I include the databases ?