Every web page has titles and subtitles - the big text that separates sections. In HTML, these are heading tags: h1 through h5. The number controls the size - h1 is the biggest, h5 is the smallest.
<h1>Page Title</h1>
<h2>Section Title</h2>
<h3>Subsection</h3>
<h4>Detail Heading</h4>
<h5>Fine Print Heading</h5>h1 is huge. You'll rarely use it - when was the last time you saw a title that big on a real website? In Salesforce, the page header and record names are typically h1 level, but Lightning handles that for you.
In LWC components, you'll mostly use h2 and h3 for section titles within your component. Think of Salesforce card headers, related list titles, or section labels in a record page - those are h2/h3 level.
Headings are about structure, not just making text big. Search engines and screen readers use heading levels to understand the hierarchy of your page. Using an h2 just because you want bigger text is wrong - that's what CSS is for. Use headings to mark actual titles and sections.