Variants
A variant is the same product in another form — a size, a colour, a finish. Each combination is priced and counted separately, so a shirt in Large / Blue can sell out while Small / Red is still available.
Stock
Set a stock count and the card keeps it honest: the status shown to visitors is worked out from the count every time the product is read, so a product cannot sit there saying In stock after the last one has gone.
Counts come down as you mark orders fulfilled, shipped or delivered — not when the order arrives. An order you have not confirmed has not left your shelf.
Collections
Collections group products so a visitor can browse a drop or a category rather than one long grid. They are managed from the Products screen.