Make is a tool to automate the steps necessary to compile complex programs.
In a nutshell, the developer(s) create a file (named Makefile) that contains rules for how the program should be built. It doesn't actually perform the compile itself; it calls other programs (usually gcc) based on what the rules specify. At a very abstract level you could kind of, sort of look at it as a shell script; an intelligent shell script.
Like synaptical said, if you'd like more information, check out the man page, or download and read the GNU Make manual.