We call it the &quot;capture&quot; operator. What it does is it captures the nth parameter of the anonymous function. Like some have already said, the number just signifies which parameter you&#x27;re making a reference to in the function body. It&#x27;s terse and comes in handy for ad-hoc stuff. I prefer verbosity and often write Elixir code with the standard syntax and find myself using the capture operator a lot more in the REPL.