The header tag, or the <h1> tag in HTML, will usually be the title of a post, or other emphasized text on the page. It will usually be the largest text that stands out. There are other header tags in HTML too, like an h2, h3, h4, etc. Each can have a lesser degree of importance on the page, but it really depends on how your HTML/CSS guy did the layout. Sometimes, they make your logo the h1 – because it’s in the “header”, but it would be best to make the h1 the title of the page or post instead.
When you want to use your logo for the h1 tag you could do that very easily with a bootstrap class called
<h1 class="text-hide" style="background-image: url('/assets/brand/bootstrap-solid.svg'); width: 50px; height: 50px;">Bootstrap</h1>