Explanation:

That is a the regex string for that sites password field. Regex is a sequence of characters used to see if an input matches a defined pattern to validate the input in code (theres also other uses but thats what being done here). Sites normally dont show the regex pattern since it is pain to parse even if you know how to write things in regex and to people who dont code this looks like a random output. Im assuming a bug exists that prints out the wrong error string so that this shows instead of the human readable one