H1 is the most important header and H6 is the least important header. User agents usually render the more important headers in larger fonts than the less important headers.

This text is in an H1 element.

This text is in an H2 element.

This text is in an H3 element.

This text is in an H4 element.

This text is in an H5 element.

This text is in an H6 element.