An English idiom with a number of meanings:
1. To understand or perceive; "He finally began to catch on to how pipe links work." See also, cotton on.
2. To gain popularity, or to become popular; "Pet rocks caught on quickly".
3. When a fire is first ignited, or when it spreads to a new area or substance, it is said to 'catch on'. "After much effort on our part, the house finally caught on fire."
More intuitively, 'catch on' can be used in much the same way as 'grab on.'