here's an example of some text content, and a variety of markups for it:
This is a headline
This is a paragraph
troff
.SH
This is a headline
.LP
.PP
This is a paragraph
.LPThe markup we're adding here looks like .SH to open a 'short heading' or .PP to open a 'paragraph', and .LP to close the open tag.
GML ~1970s
:h1.This is a headline
:p.This is a paragraphThe markup we have here are just macros at the start of a line to tell us which mode to shift into, :h1. begins a headline, :p. begins a paragraph
SGML from ~1980s
<head>This is a headline</head>
<p>This is a headline</p>
HTML from ~1990s
<h1>This is a headline</h1>
<p>This is a paragraph</p>Markdown from 2000s
# This is a headline
This is a paragraphEtc, all various different ways of marking up the text content in ways that describe what that text content actually is and means