Minifying, or minification, is where you remove unnecessary characters from your code, whether they might be whitespace (such as indentation), code that isn't ever used, comments in your code or long names for variables that can be replaced with something shorter.
Minification of your code results in it taking up less space, making it faster to send from a server to a client, as well as using less bandwidth in doing so. This improves the user experience on your site as it can load faster.
You should only minify the code that you are distributing though, not your source version that you are writing, as minified code is harder to read and understand, making debugging more complicated. Providing a source map helps with this, as it maps the minified code back to the original source code, allowing production errors to be mapped to the correct bit of code in the source version.