Adding text between header and posts

How much text do you want to add? Would it remain the same or would you need to change it regularly?

If you only want to add a small piece of text, and it wouldn’t change regularly, the very easiest way to do it would be using some custom CSS.

.site-branding::after {
     content: "Hello!";
}

Other methods are more complicated, and depending on your requirements, could entail hard-coding the text or adding a new widget area in a child theme.

If you let me more what you had in mind I can make further suggestions.