Beyond the basics: fr units, minmax, auto-fit/fill, spanning, named template areas, auto-flow: dense, alignment, and named lines.
1fr 2fr 1fr — middle column takes twice the free space.
repeat(4, 1fr)
repeat(auto-fit, minmax(120px, 1fr)) — tracks stretch to fill the row.
repeat(auto-fill, minmax(120px, 1fr)) — empty tracks are kept, items don't stretch to fill them.