It's essentially a small extension of HTML via JS, that allows all elements to fetch HTML from a server on events, and replace a target elements inner/outer html/append to it. From their page [1]:

motivation

    Why should only <a> & <form> be able to make HTTP requests?
    Why should only click & submit events trigger them?
    Why should only GET & POST methods be available?
    Why should you only be able to replace the entire screen?

By removing these constraints, htmx completes HTML as a hypertext

[1] https://htmx.org/