You'll get a drink, entree, and a dessert.
An entree could be a single item or a combo plate, for example if it's listed on the menu board as "sandwich & fries" (fruit or fries or chips might be youur side choices too) that is one combo meal. If the menu board lists just "sandwich" that's all you get, though I'd guess a plain sandwich might come with a pickle spear or other plate decoration. At Cosmic Ray's in MK the "chicken & ribs combo plate" with veg & mashed is considered one entree and is listed as such.
Keep in mind some places will let you sub an extra bottle of water or a small side item for the dessert part, varies it seems at the whim of the CM. Just ask the cashier. At places where you get the food first then pay (most resort food courts and the Seasons in Epcot come to mind) the CM's at the stations pretty much know what's included on the plans.