Hi!
The idea is to find the cache, log your name into its logbook, and then rehide it back where you found it. Often there are small toys or beads in the cache and those you can trade something small that you brought with you for one of them. I carry some semi precious stones and marbles and those are usually acceptable trade items. The more valuable trade items we try to trade value-to-value. Most stuff is dollar store level trades, stuff for kids.
There are also travel bugs and coins that are traveling from cache to cache to accommodate mileage and fulfill missions. Those can be picked up with out trades but need to be placed into a different cache later to keep them traveling. They aren't for keeping.

It is free to sign up at www.geocaching.com and there are videos there to help explain more. I hope you give it a try!
Thank you!