Aircrack-ng is an 802.11 WEP and WPA-PSK keys cracking program that can recover keys once enough data packets have been captured. The objective is to capture the WPA/WPA2 authentication handshake and then crack the PSK using aircrack-ng. The (monitor mode enabled) message means that the card has successfully been put into monitor mode. This disables the wireless card from connecting to the internet, allowing it to focus on monitor mode instead.”]
Source: https://hackercombat.com/10-steps-to-hack-wi-fi-password-of-someone/