Is VLOOKUP really the best method to match data cells for products?

I've tried to many times to understand how to use VLOOKUP and I just can't seem to grasp it.

I've got a list of products with skus in one sheet. In another sheet, I have data for SOME of those same skus (spectifically, skus that are the slowest moving) that tells me how many days it's been since that sku sold.

I want to create a new row in the first spreadsheet with a column denoting how many days it's been since that product sold, so that I can then use that spreadsheet to make a reorder, but not reorder items that haven't sold in X number of days.

I've tried many times to understand if VLOOKUP is right for this, but again, I just end up frustrated because I can't seem to understand what it does, or how to get the data I want from the specific column I want into the matching column for the correct sku on the other sheet.

Would anyone be able to help me understand how to do this?