Catch on : Phrasal verb of catch.
Meaning : To understand and to become popular/to be well known.
To understand something in a stiuation where you have very less information about a subject. it carries little more meaning than 'understand' because you don't have much explation of a situation.
Example (To understand): Before I catch on the topic of discussion, my manager moved on to next topic. (catch on implies somewhat delayed understaing of something)
Example (To become popular): Samsung mobile was known to city people only one year before but after aggressive advertisement it is caught on in rural areas as well.