RDP (Remote Desktop Protocol) is the Microsoft protocol that lets you control a Windows computer or server from a distance, seeing its desktop as if you were sitting in front of it. It is very useful for support and remote work, and it should be protected with a VPN, MFA and strong passwords instead of being left open to the internet.
Go deeper on the TecnetBlog: What is RDP
remote desktop protocol, remote desktop, port 3389, remote access