Definition: a mild powdered acid (potassium bitartrate) that stabilises whisked egg whites and helps stop sugar crystallising.
Cream of tartar is a quiet helper. A pinch whisked into egg whites lowers their pH, which makes the foam more stable and forgiving — your meringue holds its peaks and its swirls better, and is less likely to collapse or weep. It also discourages sugar from crystallising, which is handy in syrups and icings. It’s the acid half of most baking powders, too.
In practice: add a pinch to the whites as you start whisking, before the sugar goes in.
Related:French meringue,crumb.