Other forms: caught; catching; catches; catched
When you catch something, you grasp it or seize it in your hands, the way you catch a basketball when your friend throws it to you.
As a verb, catch describes grabbing and holding on. Adding the preposition on means you understand something, like when you catch on to your little brother's scheme to get out of doing chores, or to describe something that's getting popular, like a pop song that catches on then becomes a huge hit. If someone tells you there's a catch, it means there is a complication, but if a person is a catch, he or she makes a good romantic partner.