Generally speaking it means an accumulation of objects gathered for study, comparison, exhibition or as a hobby.
When you talk about a collection on your online store it means a group of products gathered into collections to make it easier for your customers to find them by category.
To make it clearer here are a few examples of collections that you might see on a store page:
- Clothing items for men, women, or children
- Items for home decor such as lamps, cushions, or rugs
- Items that have a certain size or color
- Seasonal products like holiday cards and decorations