Costco is an acronym for what?

Costco is a warhouse based store the specializes in the selling of case goods and bulk items.
Costco is a warehouse sized, multi-location store in the United States. It sells large quantity of products in bulk sizes.

For instance if you were to go to a regular local grocery store and purchase a dozen eggs, a bottle of laudry detergent and a can of tuna, in Costco you would end up with:

36 pack of eggs

3 gallon bottle of detergent

6 can pack of tuna or a jumbo industrial size can mostly used in restaurants

The prices, because you buy in bulk, tend to be a little cheaper as well.