You are significantly tougher than normal.
Prerequisite: Base Fortitude save bonus +2.
Benefit: You gain a number of hit points equal to your current Hit Dice. Each time you gain a HD (such as by gaining a level), you gain 1 additional hit point. If you lose a HD (such as by losing a level), you lose 1 hit point permanently.
Special: A fighter can select Improved Toughness as one of his fighter bonus feats.
Source: Complete Warrior, page 101. Reprinted in Libris Mortis, which left out the Special entry and Fighter descriptor.