Kendo is a suite of "components", sometimes called "widgets", for various web frameworks (Angular, React, Vue, and jQuery)
There are a lot of important features that many modern web applications need, such as styled drop down lists, beautiful forms, special kinds of date validation, charts, grids - very standard things you see a lot. Many of these are hard to create "well" because of the many concerns to do with security, browser compatibility, styling, usability, and performance.
Instead of wasting time trying to write these components themselves, many developers purchase suites like Kendo to just plug in to that space and move on with other parts of the program that cannot be "packaged" so easily.
So it's essentially a tool kit for web applications of commonly needed behaviors and features that are possible to write yourself, but not very cost-effective to.