What do square brackets in class names mean?

Question

I saw here square brackets that are used in class names:

Answer 1

That is most likely used by some sort of validator or validation library. The class here means that validate this field denoted by validate keyword and then:

required it is required field
custom validation type; allow only letters
length should be between 0 to 100 chars

Well, this information is used by the jQuery validation library you posted the link to :)