Words, phrases, symbols, designs (or a combination of those things) that distinguish a product or service from other kinds of products and services. The mark has to be unique: it can't look like, sound like, or mean the same thing as an existing mark (if it does, that's called "likelihood of confusion").
Unlike patents, you don't have to apply for a trademark right away: sometimes it takes years to achieve the brand recognition you need to argue that your logo/color/name/etc is distinct enough to be trademarked.