"Looking up" some information means searching for somewhere like:

• on the Internet
• in a book
• in some files

Most of the time, you use the phrase "look ___ up" when you were successful at finding the information that you were looking for. So you'd say something like this if you weren't successful:

I tried looking it up online but I couldn't find anything useful.

Like a lot of phrasal verbs, "look up" can be separated by words like "it", "them", "us":

Don't believe me? Look it up.

But longer words come after "up":

I looked up how far away the closest star is, and I was amazed at how far it was.