Learn CSS Grid
This site was created by Jonathan Suh as a quick reference for learning CSS Grid. There are 17 examples with explanation of what each does ad illustrations. The explanations are brief, and the illustrations make it easy to understand.
Grid By Example
This site is much more comprehensive that the source sited above. It boasts, “Everything You Need To Learn CSS Grid Layout.” It has videos, examples, and resources. There is a get started section that has the definition of grid along with links to articles and blogs as well as example code. It covers placement and sizing and much more. This site has a very large number of resources from MDN and other places.
CSS -Tricks
This site has 2 columns. One has the parent properties and the other shows the child properties. It also contains illustrations and example code. The explanations are simple. The illustrations enhance the explanations. This site also contains help with other CSS properties, so it is the most useful of the three.
Summary
There are many resources available on the web. Some are basic and others are very detailed. All one has to do is search and there is help for any level of understanding. Of the 3 sited here Grid By Example looks like it is worth spending time exploring.