The CSS `grid-template-columns` property is a fundamental part of the CSS Grid Layout, which allows developers to create two-dimensional grid-based layouts using rows and columns. This property defines the number and size of columns in a grid layout, accepting various values such as fixed lengths, percentages, fractions (fr), and functions like `minmax()` and `repeat()`. It provides flexibility by adapting to the content or container size using values such as `auto`, `min-content`, and `max-content`. Additionally, `grid-template-columns` can be animated, enabling dynamic transitions in grid layouts. Combining this property with others like `grid-auto-flow` allows for more granular control over item placement, while understanding common pitfalls such as mixing incompatible units can prevent layout issues. The property has been widely supported across major browsers since 2017, and while some experimental features remain under development, the existing functionality offers extensive possibilities for creating responsive web designs.