Sometimes we all try to do something that is too big or too complicated for us to do, i.e. we try to BITE OFF MORE THAN WE CAN CHEW. It has nothing to do with food, it can relate to anything we want to do.
For example, you decide to organize a party for 50 people all by yourself and your friend says, "Don't bite off more than you can chew, ask someone for help!"
If you take too many classes at college and you can't prepare well for the tests, you also have BITTEN OFF MORE THAN YOU CAN CHEW. You can also say that you overextend yourself or you take on too much.
At the end of the calendar year some people want to complete all the tasks and run all the errands they haven't been able to finish during the year, but remember not to BITE OFF MORE THAN YOU CAN CHEW.