What HTML escape does
HTML escape converts characters that browsers treat as markup into entity form so text displays literally instead of executing as tags. When you escape html online, angle brackets become < and >, ampersands become &, and quotes become " or '.
This is not encryption — anyone can unescape entities — so treat escaping as a presentation and safety layer for text contexts, not a way to hide secrets.
Basic mode escapes the five characters that matter most in HTML text nodes. Full mode also emits numeric entities for non-ASCII code points, useful when you need predictable ASCII-only wire format.
Escaped output grows longer than the original. A short escape string with emoji may expand noticeably in full mode because each non-ASCII character becomes a &#...; sequence.