Privacy
The QR code is built by a JavaScript library right on this page: the text and Wi-Fi password you enter are never sent anywhere and never stored.
About error correction
A QR code contains redundant data, so it stays readable even when damaged. Level L recovers up to 7% of the area, M - up to 15%, Q - up to 25%, H - up to 30%. The higher the level, the denser and larger the code. For printed stickers and business cards M is usually enough; pick H if the code will get scuffed or partially covered.
Wi-Fi
The WIFI:T:WPA;S:name;P:password;; format is
understood by iPhone and Android cameras: point the camera at the code and the
phone offers to join the network. Handy to print out for guests.