Either a small bag made out of fabric that is tied at the top with a ribbon, or a colored paper bag with stringed loops at the top for holding. The bag will contain a gift for one person to give to another as a surprise.
Don't confuse this with a sack, often associated with Christmas and Santa Claus. A sack is one large bag that can carry multiple wrapped gifts, whereas this is a single bag with a gift inside it. An example with both a gift bag and a sack clearly present is Danbooru post #374610.
