If you catch on to something, you understand it, or realize that it is happening. He got what he could out of me before I caught on to the kind of person he'd turned into Wait a minute! I'm beginning to catch on
understand, usually after some initial difficulty; "She didn't know what her classmates were plotting but finally caught on
If something catches on, it becomes popular. The idea has been around for ages without catching on
understand the situation, comprehend what is going on; become popular, become widespread; get stuck on, hook on