With a free account, you can upload up to five scores. After that, the oldest one is hidden if you upload another.
With a PRO account, you can not only share unlimited scores (and re-discover the older ones that were hidden when you reached the free account limit), but also access a number of advanced score management controls.
For the full list of features, see https://musescore.com/upgrade. You can subscribe to a PRO account on a monthly or yearly basis. Either way, the first 30 days of a PRO account are free, and you can cancel your subscription at any time in the first month at no cost.
If your PRO account expires, don't panic! None of your scores will be deleted. This just means that only the most recent five scores are accessible. If you upgrade again, all of your scores will become available to you again.