This is an interesting term and varies in usage depending on your climate. It describes plants that are perennial in warmer climates, but can be grown as annuals in colder climates. These plants can be categorized with annuals because they bloom the first year from seed. They are termed half hardy because although they can handle frost, they can't survive extremely cold winter weather. If you live in an area with mild winters, they may be perennial for you. Osteospermum is an example of a half hardy annual, as is snapdragon (Antirrhinum). And in a mid-way location, you may find they survive and overwinter successfully for you during especially mild winters, but not all years.