Base saves are ratings of how capable a creature is at (partially) avoiding certain forms of attack on the basis of classes obtained. To these are added bonuses (e.g. from feats and abilities) to get a creature's saving throws. For example, consider a level 16 barbarian / level 14 rogue who had a 12/8 class split at level 20. This character's base fortitude save from pre-epic levels is 10 (8 from twelve barbarian levels and 2 from eight rogue levels). To this is added 5 for his epic levels (character level 30), resulting in a base fortitude save of 15.