Selector problem? _html/css_WEB-ITnose

Ask again, what does li.over mean?
.over should be a li class name. . . .
Example:

• xxx




I just started learning, this was written by someone else, I didn’t pay attention to it, I wrote this.className = "over" in js, I didn’t pay attention to it.
I know about the class selector, mainly because I didn’t see over in li at first, and thought it was a selection method I had never seen before.
Thank you for your enthusiastic help.