It is software that allows to navigate on the Web.
Google chrome have created a new technology to replace cookies, it is called FLOC. This technology is a serious breach into privacy, since it records in the browser a profile based on your history. It is not possible to deploy it inside EU, because of the GDPR. You can check if your browser uses FLOC at : https://amifloced.org/
A minimal and very experimental web browser in Rust https://github.com/twilco/kosmonaut
A minimal web browser based on gecko engine https://www.palemoon.org/
- story of the building of a web browser in Rust : https://joshondesign.com/tags/browser