How can I add a hover effect to icons with CSS?

[Itarlydar]

I am updating my website and used this article How can I display social icons on my website with HTML and CSS? to add some social media icons to my website.Now I am wondering, how can I achieve a hover effect, i.e. change opacity). I know this should be only a few lines of code but I have no idea how to achieve that (CSS beginner)