Creamer is a liquid made with oil and sweetener that approximates the taste and mouthfeel of milk and sugar. Or really a flavored syrup, not sugar. They often have hazelnut or vanilla flavoring added. You would use it by pouring it in your coffee, at about a 1:1 ratio with how you would use milk.

I'm surprised you associated it with espresso. Creamer and drip coffee machines go together. Milk and espresso machines go together. I've seem drip coffee machines with real milk, but I've never seen an espresso machine with just creamer.

It tends to exist primarily in office environments, because it can stand not to be refrigerated and was the first lactose-free option. Therefore it was easy to buy one bottle and solve the problem of "stuff to add to coffee".