Once you've created collections, you can apply them to your products.
First, go to your Dashboard > Products > View Products > Edit Product.
Scroll down until you find Advanced Settings and you'll see all the labels you created in the box under Product Collections.
Click on as many labels as you want to apply them to the product.
Once you click save, all the selected collections will be applied to the product. This setting is particularly used in your WebStore so that you customers can search your products by your collections.