Tcpdump is a command-line packet analyzer tool that allows users to capture and analyze network traffic in real-time. It is commonly used for troubleshooting network issues, monitoring network activity, and analyzing network protocols. Tcpdump captures packets that are transmitted and received over a network interface and displays them in a human-readable format. It can filter packets based on various criteria such as source and destination IP addresses, port numbers, and protocol types. Tcpdump is available on most Unix-based operating systems and can be used with various options and filters to customize its behavior.