A dot in CSS means you’re referring to an HTML Class. Note that this is different from the OOP Class in scripting languages. Think of HTML classes as classrooms. You can have multiple divs with the same class name, each will be affected when you style that class name. Just like students follow the same rule when applied to a classroom.
In some specific instances, ID is used instead of class. # is used in CSS instead of a dot. The only difference is that in ID, each element should have a unique ID.