As the name implies, a plug-in range hood is a plug-and-play kind of product. These kinds of range hoods have a 3-pronged plug that you'll plug into a 120-volt outlet in the upper cabinet.
If your range hood doesn't come with a plug, then it's hardwired. That means you'll have to physically connect it to your household wiring.
An alternative is to buy a power cord adapter kit to plug it in to use it.