What is superfine sugar?

This is referred to as "bar sugar" by some people. For decades, bartenders have been using this stuff in cocktails as it dissolves quickly.

Thanks everyone ?

It Canada it's sometimes called "Berry Sugar" but the dry blender method also works great.

Sold at most grocery stores if you don't want the trouble of making it. Sold as "super fine". Look for a box rather than a bag. C & H markets theirs as "Baker's Sugar".

I purchased mine at a grocery store. I had never heard of it before I found it listed as an ingredient in a recipe I wanted to make, but it's pretty standard.

As others mentioned, making your own is easy.
I like to make a cup at a time, often flavored with dried (edible) lavender ground in. It's lovely sprinkled on fruits ( here in the frozen north) this time of year, to bring out some flavor. I've tried using a bit of dried orange or lemon peel as well.

It is extremely finely granulated sugar. It's easy to make your own by buzzing up regular granulated sugar in your food processor.

as oldunc says. Often used to sweeten cold beverages, (we use it for lemonade) where regular granulated sugar won't dissolve. For that purpose, you could also make simple syrup. If you make your own with a food processor, pulse it but don't grind for too long or it becomes confectioner's sugar.

Just what it sounds like-very fine granulated sugar; it's used where dissolving the sugar is a problem. You can usually just use granulated (with care)- you could run it through a VERY dry blender.