What is a grid fin?

A falling booster has no wings and no runway, but it does have air to work with. Grid fins bite into that air and tip the rocket left, right or around — the same job as the feathers on an arrow, except these can move.

The lattice shape gives a lot of surface in a small, foldable package, and it keeps working at supersonic speed where a flat plate would stall.

Falcon 9’s are titanium and left unpainted and uncooled, so they survive re-entry heating on every flight without being refurbished. That is what made unlimited reuse practical.