You are good at climbing, jumping, swimming, and other such physical activities.
Check: you can climb up or across a surface (as per the Climb skill description in the 3.5 PHB), jump up or over an obstacle (as per the Jump skill description in the 3.5 PHB), and swim in water (as per the Swim skill description in the 3.5 PHB).
Special: 5 or more ranks in Athletics allows you to retain your Dex bonus to Armour Class while climbing.
Synergy: 5 or more ranks in Athletics gives you a +2 bonus to Acrobatics checks involving jumping. Conversely, 5 or more ranks in Acrobatics gives you a +2 bonus to Athletics checks. These bonuses increase by +1 for every additional 10 ranks (that is, +3 at 15 ranks, +4 at 25 ranks, and so on).