CSS-Tricks
CSS-Tricks provides numerous pieces of information in regards to CSS grid, including the introduction, basics, terminology, and properties that apply to both parent and child elements. The visuals present for each property allows a user to easily see how that property is affecting a grid container or item, making it simple to only modify desired aspects.
CSS-Tricks: Complete guide to gridW3Schools
W3Schools allows the user to learn about the CSS grid, look into the browsers that support it, and learn the various properties and workings of the CSS Grid. W3Schools offers a variety of example code areas, where the user can modify values to make their own grids on the site, as well as test certain conditions to see how the grid will react. Overall, the website allows the user to experiment with the grid properties and values which aids in the understanding of how grid works.
W3Schools: CSS Grid LayoutMDN Web Docs
The mdn web docs provides an extensive amount of information on how a CSS grid works, the various properties, functions, and data types work, as well as providing multiple guides ranging from basic grid concepts to using sub-grids. In addition, there is a built in tool on the site that allows users to modify some HTML and CSS to make their own grid and experiment with different properties, structure, and values.
MDN Web Docs: CSS Grid LayoutSummary
While the three sites listed above are great references for a CSS grid, there are numerous other sites that provide useful information, guides, and even designs and pre-built patterns that you can use for your own site.