Can I read .acsm files?

Forgive me for my complete and utter obliviousness, but I have literally never read a single ebook in my life.

I downloaded an .acsm file on my PC. I downloaded calibre after some searching on the web. Now I can't seem to open the file. Am I doing something wrong? I prefer to support open source software, so I want to avoid proprietary solutions unless I absolutely have to.