H1, H2, H3 are standard attributes applied to text set out in long-accepted HTML conventions.
They change the appearance of text which you designate as a header (be it as large as usually possible, H1, or the smallest possible, H6) - typically by adding weight to the font (bold or strong), changing its size and usually adding more vertical space both above and below it, so as to make it stand out - as headings almost always do.
In fact such Hx tags are applying such styles to your browser’s default stylesheet, which most non-developer users never (need to) change, or even know about.
Developers familiar with the purpose, syntax and methods to change their own custom CSS can effectively overwrite default definitions - by making an H1 even bigger, or having H2 appear in red, or underlined etc.
RW developed the convention of a Title and a Slogan for sites which want to use them. By their nature they too are generally bold and larger than body text.
Most themes allow you similarly to overwrite the default styles which the theme’s designers have applied to Title and Slogan when you buy the Theme; these too you can change in a RW Theme’s settings using custom CSS.
To best understand what works, why it works the way it does, and how you can gain complete control over it, you may want to separate the way that Title/Slogan behave - often in a site’s banner/header area - on the one hand; and Hx tags in text on the page on the other.
Do ask if you have other questions… good luck!