The prowess of player characters is measured in levels. Levels are earned through the accumulation of experience points. A separate table for each character group (shown in Chapter 3 of the Player's Handbook) tells how many experience points characters of that group need to attain each level.
When a character earns enough experience to attain the next level for his character class, he immediately gains several benefits (unless the optional rules for training are used). The character gains an additional die of hit points, or a set number of hit points at higher levels. These are added to both his current total and his maximum number of hit points. The character may or may not improve in other abilities, including combat and saving throws, dependent upon his character class.