Your experience rate simply tells you how much experience you actually
get from various tasks.

  For example, if your experience rate is 80% and a monster is worth
100 experience points, you'll only be getting 80 experience points of that
100 if you kill it. The race you choose determines your experience rate, 
but several features can reduce it permanently, such as dsleep, molt or 
auspice. Your experience rate also affects the experience points you earn
by exploring.

  In parties, other factors such as your level compared to other party 
member's levels and possible slayer statuses affect the amount of exp you
receive from kills.

see also: help experience points, help exp rate reduction, help party exp