An apron is an article of clothing covering the wearer’s front, worn over garments to protect them (though sometimes worn for decoration as well). It’s a period artifact, with examples in illuminations dating to at least the 14th C., but we’ve no examples of the apron’s use as an independent charge in period armory.
Though styles of apron were not distinguished as “waist-aprons” or “bib-aprons” in period, the distinction is made in Society blazon for the sake of the artist. The illustration, based on the Luttrell Psalter, shows a waist-apron.
Morgan Donner bears: Gules, a cloth waist-apron strings loose argent.
Ása in svarta bears as a badge: A bib-apron azure.