Catnip, as many cat owners know, is an herb that you can buy at many pet shops and actually it's embedded in a lot of cat toys. It acts as a bit of a drug for cats. You'll find though that not all cats respond to catnip.
What is called the "catnip effect" is only in some cats. When you do give catnip, the important thing to keep in mind is to only give a tiny bit until you know how your cat responds. If your cat seems to be having a good time, it's lolling around and excited, playful, or just really wistfully sleepy, then you can continue to use catnip. If your cat seems more irritable or aggressive or fearful when you give catnip, then you shouldn't give that cat catnip anymore, because it's not the right option for that one. Some people use catnip to train with, and you actually can do that quite easily. You just use a tiny pinch as a reward.
Just remember that if you use catnip as a reward in a training session, your session might be over relatively quickly. And that's some information about catnip.