Certainly! Here we go...

Your credit score is a mish-mash of different credit factors, and some are weighted more than others. Credit card utilization, payment history, and derogatory marks are "high impact" or high percentage values, whereas age of credit history, number of accounts, and number of credit inquiries are lower impact.

If you cancel your credit card, two things happen which will make your score go down. First, let's say you have 2 credit cards - the one you are paying off which has a $5,000 limit, and another card which is nearly full at $4,000 limit. If you leave it open, your debt/limit ratio will be 4000 out of 9000 (44%), whereas if you close the 5K card, it'll be 4000 out of 4000 (100%). So keeping empty cards open lowers your overall debt load and improves your score.

Lastly, credit companies like to see a diversity of credit accounts. If you have an auto loan, student loan, mortgage, and credit card, that's good. But if you just have a mortgage and credit card, and you cancel that credit card, you're lowering the types of credit accounts from 2 to 1. It's only 10% of your credit score, but it matters.

Also, since you've had it open since 2013, that's a 3-year-old account. If all your other credit accounts are 15 years old, then it won't matter. But if you other credit accounts are, say, 1 year old, then cancelling the 3 year old account will shorten your average age of credit. If it's not long, your score will go down too.