learncssgrid.com
This site gives a description of the most commonly used grid properties, with illustrations for each one. It is perfect for beginners or anybody else looking to understand the basics.
Mozilla Developer Network
MDN provides a more comprehensive explanation of each property, along with code examples and more illustrations when necessary.
CSS Grid Generator
This grid generator is an interactive tool that allows you to experiment with creating your own columns, rows, and gaps. You can immediately see the changes made to the grid based on your own input.
Summary
Grid is more optimized for two-dimensional layouts, which alongside flexbox provides the modern approach for laying out web pages. Developers have complete control over the height and width of any grid item inside of its grid container.