There are 6 heading elements, each one displaying a different size, with h1 being the biggest and h6 being the smallest. You can have more than one heading that has the same heading tag as it does not number the headings but rather defines their size. The options are the following:
The number effects the importance of the heading and should be grouped in that way. For instance, in the case of a book, you would make all the chapters titles have the same heading because they are of the same importance. (and by extension, they will all have the same size)
The heading element gives its value a bigger size, specifically: and it boldens the text.
the size of the heading can be changed, the color, border, etc.