If you try to put your cat into her carrier so you can take her to the vet, she's likely to resist. She'll squirm all over the place, trying to stand strong against you, so that she doesn't have to go into the carrier. You really have to admire her resistance. She's very strong.
If you're having a great time at the pool or the playground, but then your parents say "It's time to go home, so get in the car," you might resist: you might run away, or refuse to get in the car, or say, "No! Let's stay! Just one more hour! Please?" Your resistance is understandable. It's hard to leave a fun place.
It's also hard to resist delicious, unhealthy foods. It's tough for me to resist French fries. They're so greasy, so unhealthy, oooh, but they're so tasty! Sometimes I resist them and choose something healthier, but sometimes I don't resist them: I chow down on those fries. Yum. French fries can be irresistible: hard or impossible to say "no" to.
Just like it's difficult to resist yummy junk food, it's also difficult to resist an urge to do something. If you really want to laugh, but you know it would be rude, you've got to resist that urge to laugh. And that's hard!