What are netstat switches?

Netstat -tlnp is the netstat switches application with arguments:

-t: Show only the TCP protocol

• -l: Show only listening sockets
  
• -n: Show numerical addresses instead of attempting to resolve the hostname
  
• -p: Show the PID of the program that is associated with that socket
  

So combined you are running netstat that will show listening TCP sockets and display the IP address and PID of the socket. From the netstat man page

Edit: The reason for using each argument:

• -t is useful if you do not want to see all protocol sockets (you can use -u for UDP)
  
• -l cleans up the output so that you will only see the sockets with the state LISTENING instead of all states (closed, established etc)
  
• -n will display the IP address and port number, this makes the results display quicker as netstat will not attempt to perform a hostname lookup which can take time.
  
• -p can help you identify which running process is attached to the socket and listening on the port