OpenID is a technology that allows you to log into new websites (such as checkin24-7) using your existing login/password from a different website. The advantage is that you don't have to remember lots of login/password pairs for each website you want to use. Another advantage is that the registration process is much shorter, there is no email verification step if you use the OpenID login in checkin24-7.
Most likely you already have at least one OpenID account. Your Google, Yahoo, AOL accounts (as well as many more) can be used for OpenID login in checkin24-7.
You can find out more about OpenID
at OpenID.net