It's just an attribute on the element. It doesn't have any meaning by itself, so it may be present simply as a red herring.

Prettified, the code is:

<img
  src="/"
  =_=" title="
  onerror='prompt(1)'"
>

In HTML, = in an attribute specifies the delimiter between the attribute name and the attribute value, so it's:

=_=" title="
^^ attribute name

=_=" title="
  ^ delimiter between attribute name and attribute value

=_=" title="
   ^ attribute value contents delimiter

=_=" title="
    ^^^^^^^ attribute value

=_=" title="
           ^ attribute value contents delimiter

And you could retrieve the attribute value if you wanted.

const img = document.querySelector('img');
console.log(img.getAttribute('=_'));

<img
  src="/"
  =_=" title="
  onerror='prompt(1)'"
>

Note that the attribute name is =_, not =_= - the final = is the delimiter, not part of the attribute name.

The "XSS" is caused only by the src and the onerror, not by anything else. Wherever you're encountering this, the =_ probably doesn't do anything at all. It could, but it probably doesn't.

<img src="/" onerror='prompt(1)'">