fr

Take a grid 500px wide with grid-template-columns: 100px 1fr 2fr and a 20px gap. The fixed column and the two gaps use 140px; the 360px that are left go to the fr columns at one part against two, so they measure 120px and 240px. fr is always worked out last, as a number of parts of whatever the fixed tracks and gaps have not used.

It is not a length you can use anywhere else. A property such as width rejects it outright. It is valid only where a grid track size is expected, which means the grid-template-* properties for the explicit tracks and the grid-auto-* properties for the implicit ones, including inside repeat() and as the maximum of minmax(). The number can be fractional, and a total below 1 behaves differently from what most people expect: 0.25fr 0.25fr in a 400px grid gives two 100px tracks and leaves half the container empty, because the fractions claim only their share of the free space.

In a 200px grid, a single 1fr column holding one word 120 letters long grew wider than the grid itself, because a plain 1fr means minmax(auto, 1fr) and the auto minimum is the content’s own smallest width. The same column written as minmax(0, 1fr) stayed at 200px and let the word overflow inside it instead. The floor, not the share, is what breaks grids: remove it with minmax(0, 1fr), then give the content a text-wrap or overflow rule for the day it does not fit.