Port 3389 is the standard port for remote desktop protocol that powers Remote Desktop Services on all modern versions of Windows.
If your system has Remote Desktop enabled, it is listening for connections on port 3389. Since this port is both well known and can be used to attack accounts, it is low hanging fruit for script kiddies and bots looking for an easy target.
Theoretically on a system that does not have an account lockout policy in place, which by the way is not a system default, the RDP protocol can be used to get the administrator password with brute force. Brute force is a fancy way of saying trying all possible passwords. If the system never locks out the account then time is the only barrier to eventually getting you password and logging in.
The first defense is to implement a good account lockout policy but that does not solve the entire problem. Any administrator of a public facing Windows web server will notice that their server is continiously attacked by bots looking for an easy target. The bots will often lock out your accounts which can be very annoying.
Besides the brute force process uses much CPU and decrease the overall performance of the VPS up to a real DOS. If a brute force attack is ongoing you will see it in the event viewer with event-id 4625.
For this reason is important to change the default RPD port by changing the following registry key:
Choose any port number and restart the server.
At this point you can connect to the server adding the port number after the server IP or host name link this: xx.xx.xx.xx:yyyy