This passive ability gives you increased resistence to the "Gravity" effect. The Gravity effect reduces your movement rate significantly, and also lowers your evasion. Gravity is a black magic spell (Red Mage only) and there are probably a few monsters that can inflict Gravity through a special attack.
With this ability, you are more likely to resist/evade the Gravity effect, and it will not last as long on average when it does take hold.
No your movment speed is the same. Like all resist pssive effects you resist a magic set on you. Flee does increase your movement speed, Resist Gravity lets you resist the black magic Gravity. Not too complicated...