So every debuff/status has a set chance to proc, right? Like 40% Stun, 60% Def Break, etc, and the game rolls a number to see whether the effect goes off.
But then the game rolls a second number against the target's Resist% to see if the effect sticks. When this roll fails, that's when you see "Resisted".
You never want your monsters stunned, Def broken, etc. so it's always a good idea to have at least 30-50% Resist especially in PvP and end-game dungeons.