Using nmap or other tools, you may have seen results that say open, closed, or filtered. Here is some insight into what this all means!
Send a SYN packet -> get RST packet.
Send a SYN packet -> get SYN/ACK.
If the packet is discarded and there is no response this is typically considered a filtered port.
nmap actually has a total of six different states it can record though the above three are the most common across all networking tools.