It is skill activation speed, and in some cases affects global cooldown; it does NOT affect individual cooldowns. Mouse-over a skill so the tooltip pops up. Most melee skills have a 2.50 second global cooldown, most spells have a 2.50 second cast time. After applying speed modifiers you'll find it'll be down to like 2.47

Skills with "true" cooldowns, Vs global cooldowns are unaffected by speed modifiers. Fundamentally the speed bonuses are so minor as to be irrelevant anyway though due to the GCD system. In order for the skill modifier to make a meanginful difference you need to clock up around 2.5 seconds worth of shaved time. if you're only shaving 0.03 of a second off each skill activation, it's equivalent to one additional skill activation every 83 skills in a single chain - ie. 1 extra skill activation every 3 and a half minutes