A Product is a item offered in the Shop app that requires no specific options to be chosen by a front-end user. It can be ordered either through the Shop shopping cart experience or through a Shop Store.
For instance, you might offer a membership or a tote bag through your Shop app without requiring any specific selections to be made by the visitor with regard to either item before ordering them.
By contrast, a product Variant is an item offered in the Shop app that requires the front-end visitor to select an option from a series of variants you define for the parent Product. These Variants are all associated with a parent Product but may have unique values for various parameters with inventory managed individually. Product Variants can be ordered only through the Shop shopping cart experience.
For instance, you might off a 20th Anniversary T-Shirt for sale that has Variant Options for Color, Gender, and Size. A selection for each of these Variant Options must be made by a visitor before ordering them.