What the hell is a ''Combo'' ?

Combo is simply shorthand for "combination." It can be a combination of a username and password, or it could also be a combination of an IP address and a port (if you are working with proxies.)

So, for example, this is a combo of an IP and a port:

137.135.166.225:8128

This is an example of a username and password:

user1:[email protected]/secure/members.html

Note that in both cases the first half and the second half of the combo are separated by a colon symbol.