what is mean "catch up on"?

I'm not sure about separable or inseparable phrasal verbs, I'm not a linguist. I can only tell you that the phrase "catch up on" means to enter into a situation where the others in your group have received knowledge or news that you have missed. A frequent use of the phrase happens when a person was on vacation for two weeks, when they return the employer will tell them what happened on the job in their absence, (example, what decisions were made), so that the employee can better do their job. This is known as "catching up on" what happened at work. The use is not limited to employment, it is also applied to various gossipy stories in a social group, like Joan and Jim broke up, Harry and Steve had an argument and aren't talking to each other, etc.

I'm not a linguist either, but "catch up" is inseparable because you can't put a direct object between "catch" and "up" or the sentence will become grammatically incorrect. It should be noted that you "catch up on" something (a task) but "catch up with" someone.

Some examples:
1. The student needs to catch up on the studying he should've done on holiday.
2. The student needs to catch up with his professor, whose wife recently had a child.

It should also be noted that even though you cannot put the direct object in the middle of the phrase, you can say something like "the supervisor needs to catch the employee up on the system changes." In this case, the supervisor is not doing the catching up but helping another person catch up.

I'll go on ahead. You can catch me up. (= catch up with me)

Okay, my bad. I'm wrong.

catch someone/something up: to go faster so that you reach the person or vehicle in front of you

We left before them, but they soon caught us up again.

catch up with:

If you hurry, you should catch up with them at the bridge.

http://www.macmillandictionary.com/dictionary/british/catch-up_1

can you explain more about difference between "catch up on" and "catch up with". I don't understood do a task or what?

Hi Maya!

Keep in mind I'm not a native speaker of English. Contrex shows one use of "catch up" that I wasn't aware of, so there may be more ways to use "catch up" that I don't know.

"Catch up on" - as far as I'm aware - generally precedes things or tasks. For example: The president has to catch up on sleep. Sleep is the task the president has to catch up on. "Catch up with," however, is generally used to describe encounters between people. For example: The president has to catch up with his daughter, as in he needs to be filled in on the details of her life that have happened since they last spoke in earnest.

"Catch up with" can also be used to express a need to reach the same standard as someone or something else. For example: The university needs to catch up with times. This example suggests that the university is old-fashioned. Another example: The US diving program needs to catch up with China's if the Americans are to contend for the gold medal at the Olympics.

Hope this helps!