The phrase "to catch up on" means to do something that you have been neglecting or postponing, in order to make up for lost time. It can refer to completing a task or activity that was delayed or put off, with the goal of bringing yourself back up-to-date or current. The expression is often used when talking about keeping up with work, school assignments, personal responsibilities, or even social obligations. For example, if someone has been busy and hasn't had a chance to keep in touch with friends, they might say "I need to catch up on my correspondence" meaning that they want to write letters or emails to update their contacts about what has been happening in their lives.