To savescum is to abuse a computer game's rules by saving before you take a risk, and reloading if you do not get the result you want. This is most often done just before encountering random drops, which may be the only way to get some ultra-rare items, or in order to evade death. It is most often found in the context of roguelike games.
'Savescum' is a negative term, indicating that it is a form of cheating, although the details and the extent of cheating are highly variable. In many cases this is a matter of going against the spirit of the rules rather than actually breaking the rules, although people have also put considerable effort into manipulating games to load 'illegal' saves. Some games, on the other hand, seem to invite this sort of manipulation, and it may not be considered cheating at all in these cases.
Savescumming is perhaps exemplified in the practice of startscumming, in which one restarts a game until one gets a character with particularly good starting stats.
The forms save-scum, save scum, and the attendant save-scumming and save scumming are all in common usage. In this writeup I have followed Wikipedia's lead.