Aircrack-ng is a wifi monitoring and analysis tool that works to check the security of the WEP (Wired Equivalent Privacy) and WPA (Wi-Fi Protected Access) connection numbers.
With Aircrack-ng you can capture information packets from a specific network. Aircrack-ng includes diverse tools, like the Airodump for example, a sniffer that is responsible for capturing the packets for monitoring.
When you have captured enough packets, you have to load them into Aircrack-ng and leave it to decypher the key and find out which network they are from. This way you can know if your network is totally protected from others.