Online Google Dictionary

catch on wordnet sense
  1. understand, usually after some initial difficulty; "She didn't know what her classmates were plotting but finally caught on"
  2. become popular; "This fashion caught on in Paris"
  3. 1 vi (to understand) rikai suru: Sono uchi rikai suru. (You’ll catch on in time.)
  4. At first, Sarah didn’t understand what Harry was doing, but she soon caught on.