What do I use this for?

edit

The very first question on the Wireshark FAQ page asks this basic question and the answer provided is as follows:

Wireshark® is a network protocol analyzer. It lets you capture and interactively browse the traffic running on a computer network. It has a rich and powerful feature set and is world's most popular tool of its kind. It runs on most computing platforms including Windows, macOS, Linux, and UNIX. Network professionals, security experts, developers, and educators around the world use it regularly. It is freely available as open source, and is released under the GNU General Public License version 2.

It is developed and maintained by a global team of protocol experts, and it is an example of a disruptive technology. Wireshark used to be known as Ethereal®. See the next question for details about the name change. If you're still using Ethereal, it is strongly recommended that you upgrade to Wireshark as Ethereal is unsupported and has known security vulnerabilities.

For more information, please see the About Wireshark page.

And from the About Wireshark page:

Wireshark is the world's foremost network protocol analyzer. It lets you see what's happening on your network at a microscopic level. It is the de facto (and often de jure) standard across many industries and educational institutions.

Wireshark development thrives thanks to the contributions of networking experts across the globe. It is the continuation of a project that started in 1998.