Everyone has one primary method of authenticating to the site. In certain special cases, people can have a secondary way of authenticating, but in most cases they have just one way.
Here, you can choose what mechanism this person can use to authenticate to the site. Most of the time, you will not need to touch this as it will be set correctly for you.
In some circumstances, you might need someone to authenticate using a different method to what they have previously used. For example, if they used to use LDAP but the LDAP server does not know about them any more, and they wish to continue to get access to their account, you could set their authentication method to another one that is available in their institution. Alternatively, you can remove them from their institution and allow them to log in with the authentication method of the site if that is an option.