From Josh Comeau's CSS for JS Developers course.
Essentially, this handy-dandy “World Famous” grid snippet creates a grid with a dynamic number of columns. Each column will be the same width, and it'll try and pack as many in as it can while staying above a specified minimum (in this case, 150px).
It's useful when we have a list of items that we want to render, like a group of cards. We want them to be uniform, and to tile nicely no matter what the screen size is, or how many items we have.