It allows computers over the internet to connect to a specific computer or service within a private network.
Well known ports: privileged category of ports that range from port 0 - 1023.
| Port | Name | 
|---|---|
| 21 | File Transfer Protocol (FTP) | 
| 22 | Secure Shell (SSH) | 
| 80 | HTTP | 
| 443 | HTTPS | 
| 3306 | MYSQL | 
| 3389 | Remote Desktop Connection (RDP) | 
It basically sends the request from internet to 3389, to your 192.168.1.105:3389.