To expand a bit on the above, in your example, it's for embedding that URL as a <script> in your page to call a function named JSON_CALLBACK.
Here's how you would use it directly in your HTML:
<script>
function JSON_CALLBACK(data) {
console.log(data);
}
</script>
<script src="https://en.wikipedia.org/w/api.php?format=json&action=query&generator=search&gsrnamespace=0&gsrlimit=10&prop=pageimages|extracts&pilimit=max&exintro&explaintext&exsentences=1&exlimit=max&gsrsearch=tesla&callback=JSON_CALLBACK"></script>
Or, how you would use it with JavaScript instead:
var s = document.createElement("script");
s.src = "https://en.wikipedia.org/w/api.php?format=json&action=query&generator=search&gsrnamespace=0&gsrlimit=10&prop=pageimages|extracts&pilimit=max&exintro&explaintext&exsentences=1&exlimit=max&gsrsearch=tesla&callback=JSON_CALLBACK";
document.body.appendChild(s);
function JSON_CALLBACK(data) {
console.log(data);
}
You may also notice callback=? if you use jQuery - it tells jQuery to replace the question mark with a random number/function name and embed the URL as a script for you like we did above. It's a common way to work around cross origin resource sharing (CORS) errors - since you're running the script directly from your page rather than from the remote server, there is no origin to cross.
Luckily, with the Wikipedia API, none of this is required. Just replace that callback parameter with origin=* and you can make cross origin requests without issue.
