Chess endgame tablebases (EGTBs) are special databases that stores all possible positions with a given material balance and their results.

Each endgame tablebase file allows the user to check what the result is with best play. For example, the KNNKP tablebase,can allow you to input a specific position with such a material configuration, and find out what the result should be with best play. Nalimov endgame tablebases (one of several types of tablebases),for example could tell you if that position is a forced win and if so in how many moves, or a draw with best play.

Most chess engines don't even need to reach the position covered in an endgame tablebases to use the tablebases. For example, a few moves before such a position, the engine calculates (but does not play yet) a series of exchanges that leads directly to a position in the tablebase. The engine will then look up (or probe) the tablebase and get the results for that hypothetical position. This should help improve play of course.

                                                (from EGTB Guide by Aaron Tay)