|Ok. IP address is like address of computer. Ping other computer's IP address is to check the availability or status of other computer. |
If your laptop and desktop are connected to network, then you can ping other computer in command prompt as shown here.
Let say IP address on computerA is 192.168.1.100, and IP address on computerB is 192.168.1.150, you can "ping 192.168.1.100" from computerB and get reply from computerA if both are connected well to network.
Hope this helps.