Teamviewer is a Remote desktop Utility, which allows you remotely access another PC on the network. When you run teamviewer, you are assigned with an ID and a password(which difffers in every session\run.) 


So, here’s the whole procedure. One person starts teamviewer and tells his ID and password to the other who inputs his ID into this input box that says “Partner ID,” and clicks “Connect to Partner,” On which he is prompted for a password and in return, he gains access to the First person’s PC.