Hitpoints (often just: hp) are of course a measure of the health of your character. In the XXX/YYY format in the Score, the first part represents your current amount, and the latter part represents your maximum. Typically once your hitpoints reach 0, you die and are returned to the Ready Room (recall).
In rare cases, it can happen that enough damage will be done to your character to put them just past 0 into barely negative numbers. The character will then progress through stages of being incapacitated/mortally wounded, and will die soon if not aided. If another player responds quickly by using the Aid skill, the character can be saved from death and will be returned to life with minimal hitpoints.
Hitpoints can be regained in 5 ways: