The CSS grid layout is a layout that mainly helps to divide the page into sections and define them in certain ways. The MDN Reference provides a few demo code sections as well as all the grid properties (including experimental ones). On top of that, included towards the bottom of the page are links to guides and other material that may help developers figure out how to implement CSS grids to have the best impact on their projects.
The W3Schools reference provides more than enough demos to view and mess with. Their description of the mdoule is "The CSS Grid Layout Module offers a grid-based layout system, with rows and columns, making it easier to design web pages without having to use floats and positioning." Of the three references, this is the best option just because of the demos available. They go into depth on the little intricacies of how the module works and which properties do what.
Like the W3Schools reference, the CSS Tricks reference is more than plentiful with its demos and code explanations. It goes into great detail regarding each grid property. This along with the W3Schools reference are great to view and even use the demo code provided to apply to your own projects as you see fit.
CSS Grid is a neat little module of CSS that provides more direct structure to a web page. Of course, it's not the only way to do that, but it is the best for making sure that structure is strictly laid out and followed by the content. That said, it can get a little complex with more and more grids being laid out and can cause some issues if you lose track while building the site and/or page. Stick to the three provided references and you should be golden!