jQuery is a JavaScript library, a file of ready-made functions that a web page loads and calls from its own code. It is maintained by the jQuery contributors under the OpenJS Foundation. It makes four common browser tasks shorter to write: DOM traversal and manipulation (the DOM is the browser's tree of page elements), event handling, animation, and Ajax, which means sending requests to a server without reloading the page.
Code selects elements with CSS selectors through the $ function, for example $("button.continue"), and then calls methods on the result: .html() replaces the content, .on("click", ...) runs a function when a user clicks, and $.ajax() calls a server address and handles the reply. The same code works in Chrome, Edge, Firefox, Safari, Android, iOS and other browsers. The library is about 30 kB after minification and gzip compression. Version 4.0 is the current release; version 3.x now receives only critical updates.
jQuery is free and open source under the MIT license, which allows use in commercial projects as long as the copyright header stays in the code.