Resistance
Resistance is virtually always a free action - it doesn't use up your turn. When someone tries to impose an effect on you, you get to resist. This is true whether you're resisting magic, poison, intimidation, or just a punch in the gut. Your natural toughness, willpower, and/or training might be enough to shrug off what they're trying to do to you.
The limitation is that if you're taking advantage of the free action, you don't get to impose effect in return. Resisting a punch is just grunting and taking it, tightening up your muscles and breathing through it, maybe dodging, but even if you win by a lot the excess is lost unless you have some special trick to change the rules. If you punch back with that roll, that's not resistance, it's opposition, and it would take a turn. Resistance defends - it doesn't attack back, which is why it's a free action.
A common example is defending against multiple attackers. Any defense that can't hurt your attacker back is resistance rather than opposition - you're just trying to mitigate their success, so it's a free action, meaning you can use it again, and again… This is why splitting your EL to put a little into active defense is usually such a good bargain in the action economy…
What's splitting? I'm so glad you asked!