About QR codes
A QR code stores text as a pattern of black and white squares that a camera can scan and decode instantly. Higher error correction levels add redundancy so the code still scans even if part of it is damaged or obscured, but they also make the pattern denser.
This tool generates the QR code entirely in your browser using the qrcode-generator library; your text is never sent to a server. Long text needs a higher error correction level's extra capacity or a shorter input to stay within the QR code's data limit.