If you're familiar with ArtMoney, MemEdit or Cheat Engine on Windows, then you know what it's used for :-).
Basically, it lets you search for variables in the memory of a running process, then refine the search further (eg: give higher, lower or fixed values). You can use this basically to cheat at games. Look for the health, money or mana values, refine the search, then freeze or set the value to whatever you want.
ScanMem is a simple commandline tool on Linux that lets you do the same. There are other uses, but this one is the most common...
