Chrome is a finish used to protect steel from rust.
Chrome, aka Chrome plating is an electro-chemical reaction that puts a shiny reflective silver finish onto a steel bicycle part. Chrome is common on 80s bikes and earlier, but fell from favour in more modern road bikes. 90s BSOs might sport chrome, as might BMX bikes or those aimed at children.
If your part has a mirror-like finish, and a magnet shows its ferrous, then its probably chromed. By comparison, a dull grey finish and a non-ferrous metal is probably aluminium/aluminum, or possibly titanium, neither of which need paint or protection.