That’s jQuery and the pound sign (#) refers to an element’s ID. It’s one way jQuery can select an element. In your example, it would select the element with the ID of “searchTerm”.
As always, remember that as a developer, your time is typically the most valuable resource. Do not focus on optimization of selector speed unless it is clear that performance needs to be improved.
Each id value must be used only once within a document. If more than one element has been assigned the same ID, queries that use that ID will only select the first matched element in the DOM. This behavior should not be relied on, however; a document with more than one element using the same ID is invalid.
If the id contains characters like periods or colons you have to escape those characters with backslashes.