Browsers treat `<`, `>`, `&`, and `"` as part of HTML syntax. To display them as literal characters you replace them with entities: `<`, `>`, `&`, `"`.
Failing to escape is the root cause of many cross-site scripting (XSS) bugs, user input ending up in HTML where it gets interpreted as markup.
Frameworks like React escape automatically. When you're hand-writing HTML or building strings, escape on output.