The MDN website has guides and examples to help you learn how to use grid. They provide detailed explanations on the properties in grid.
CSS Tricks provides a brief introduction to CSS grid and teaches you CSS Grid basics, important css grid terminology, and CSS Grid properties to help you understand grid. They are very detailed with their guide.
Just like CSS Tricks, learncssgrid is a very detailed guide to learning CSS grid. They have great examples along with pictures to help you visually understand grid.
In summary, CSS grid is a very powerful tool that allows for two dimensional layouts to be created on the web.