You can create your own units for your products. For example, you might sell drinks by the bottle or shoes by the pair.
You can find the units under Settings > Shop > Scale Units.
The units are used when creating or editing products. You can select the units in the drop down menu for each product.
You can do this in the tab Specifications.
Generally, the unit is displayed in the storefront. For example, in the supplied Responsive Theme it is displayed in the listing and in the product details.