Header with Bottom Border

This Tailwind CSS code creates a header section with a bottom border. The div has a white background, horizontal padding of 4 on mobile and 6 on desktop, and vertical padding of 5 on mobile and 6 on desktop. It has a bottom border of 1px gray. Inside is an H3 element styled as semibold base sized text in dark gray. Overall it provides a clean header separated by a border.