Hello there, biplob,
Stock keeping unit (SKU) is used to track variation of your inventory items. It's an alphanumeric code that has key characteristics of the product's color, size, type, etc. For example, the SKU for a red v-neck shirt, size 6, may read "VN-RD-06".
In QuickBooks Online, you can create SKU when you add inventory items. You can check out this article to know more: https://community.intuit.com/articles/1776324.
You can also use reports to see your sales and inventory status: https://community.intuit.com/articles/1790411.
QBO has a lot to offer, post anytime if you have other questions.