RDP operates on port 3389 by default. Hackers constantly scan the internet for open 3389 ports and try to brute-force the password. If you leave your server unsecured, it will be compromised.
This sounds obvious, but "Password123" is still common. Use a generated string of 16+ characters.
Changing the listening port from 3389 to something random (like 49231) stops automated mass-scanners. You can do this via the Windows Registry (RegEdit):
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Terminal Server\WinStations\RDP-Tcp\PortNumber
If you have a static IP at home or work, configure the Windows Firewall on your server to only accept connections to the RDP port from your specific IP address. This makes it impossible for anyone else to connect, even if they have your password.
For enterprise security, do not expose RDP to the public internet at all. Set up a VPN (like WireGuard or OpenVPN) on the server. Connect to the VPN first, then RDP to the local IP address.
Microsoft regularly patches RDP vulnerabilities (like the famous BlueKeep exploit). Always install security updates immediately.