What is Xss?
1.
cross site scripting.
the website suffers from an xss vulnerability allowing remote javascript execution.
See
2.
XSS = Cross Site Scripting
XSS allow you to execute JavaScript,to steal cookies,redirect,insert malicious script in a vulnerable site.
just try this on some site's
<script>alert('xss')</script>
or
"><script>alert('xss')</script>
and all bullshits to bypass filters
See