Bundlephobia is a website and npm package that allows developers to search for the size of any JavaScript package before adding it to their project. It provides information on the size of the package, as well as the size of its dependencies, to help developers make informed decisions about which packages to use and how they may impact the overall size of their application. Bundlephobia also offers badges which can be added to your GitHub README.md file to show the size of your package.