An activeX componnent is something like a tool that you can share with different windows applications independent from the language in which the programm has been written (VB, VC, etc...)
For example, when you make a copy/paste action from an explorer window to another application (FTP programm, Word, etc...) an activeX control works in background working as a bridge between the two applications.
I hope this helped a little bit. If you want to know what an activeX is really about, look at ATL a library similar to MFC, but designed for ActiveX.