To have Swiftness (X) is to be X% faster than normal.

This only affects Combat Speed; that is, how fast you can run in battle. Your other movements aren't boosted similarly, or at least aren't controllable enough to be useful in combat or anything else.

This is not to be confused with Quickness, which provides more useful speed but is quite exhausting.

Swiftness is a buff shared between Nature and Air:

• The Bless provides Swiftness (30), along with a +1 boost to Defense (DEF), and is stackable. It requires 3 on your Pretender.
• Three Enchantment spells provide Swiftness (100); these are Windrunner (a self-buff) at Research Level 1, Gift of the Hare (AOE 1+) at Level 2, and Haste (AOE 25) at Level 4. The first spell requires 1, the second requires 1, and the third spell requires 3 and 1 (or, if you're desperate for speed, 2 and 2).
• Some items grant Swiftness, too.

For stacking purposes, you should know that the three spells do NOT stack with themselves or each other.