A man-in-the-middle attack happens when someone secretly places themselves between two parties that are communicating, for example between your laptop and your bank's website, to read or change what they send. It is common on unprotected public Wi-Fi and is prevented with encrypted connections such as HTTPS and VPNs.
Related term in this glossary: Packet sniffing
MitM, MITM attack, adversary in the middle, AiTM, interception attack