A combo update is an updater that includes all updates (10.5.x and security updates and ...) since 10.5.0 came out.
For example if you have 10.5.1 installed and don't want to go the route of installing 10.5.2, 10.5.3, 10.5.4, 10.5.5 and then 10.5.6, you just download the combo update and install that.
Or you just simply use Software Update, which downloads the specific update package for your Mac. The Combo Update includes also all machines, so a lot of redundant code for you.
Btw, what do you mean by "is it to late"?
You can update anytime you want.