Take a look at your bill. Most bills break it down. Lets say your minimum payment is $30 on that $300 outstanding balance you owe. Let's further assume for simplicity that the interest rate is 10%. That means you pay $30 a year in interest on an outstanding balance of $300. But it's prorated for the billing period, which is month. So your monthly interest on $300 at 10% a year would be roughly $2.50. So if your minimum payment is $30 the first $2,50 of that payment is interest. The remaining $27.50 reduces your outstanding balance to $272.50. Now, your available credit that you can spend on that card is $27.50.
Whereas if you send in a $200 payment, $2.50 of that payment is interest and the remaining $197.50 pays down your outstanding balance. In this case, you now have $197.50 of credit available for you to spend.
May I suggest you seek out and enroll in something like a basic "managing credit" course in your local area? What I covered above is really basic stuff that I learned in high school back in the 70's. But for over 20 years now, not even colleges offer this basic stuff. It's not like you learn this stuff through osmosis. Credit can help you if managed correctly. But on the other side of that coin, credit can ruin your life forever if not properly and responsibly managed. No, I'm not inferring you're not responsible. But how can one expect you to manage responsibly if you were never taught how to manage in the first place?