auto-FIT fits the items to the row; auto-FILL fills the row with empty slots.

Both keywords create as many tracks as the width allows. They differ when there are fewer items than tracks. auto-fill leaves the unused tracks in place, so the items stay near their minimum size. auto-fit collapses the unused tracks to nothing, which lets the items grow into the freed space.

Read it once, then a question tests whether the trick stuck.