When two things correspond, they match up or are equivalent to one another. You might come up with a code in which numbers correspond to letters of the alphabet. Correspond can also mean to send messages back and forth. You can use your code to correspond with your pen pal.
The word correspond can mean "communicating back and forth," like using email to correspond with a far-away cousin. It can also mean "be alike," as in having friends whose interests and personalities correspond to your own. Finally, it can be "be equal to," as in getting a score on a standardized test that corresponds with those of students taking the same subjects as you but who live in other states or countries.