Heading tape is stitched across the top of a curtain and drawn up on its cords to form the pleats. The tape you choose decides the look entirely: pencil pleat gives an even gathered row, pinch pleat gives grouped folds, and a simple gathered tape suits informal curtains.
Each style needs a different fullness — pencil pleat typically wants around two and a half times the track width in fabric, so choose the tape before cutting, not after. Knot the cords at one end and leave them loose at the other so the curtain can be let out flat for washing.