There are several elements. His arm strength is an obvious one. A hot qb can throw the ball deep down the field with great force. His pocket presence is another. He can sense when the defense is closing in and either step up in the pocket or roll out to avoid being sacked. And his connection with his receivers, like knowing their running routes and timing, is also an important part of showing his talents.
One possible element is his passing accuracy. A talented qb can hit his receivers right in the hands even when they are well - covered. Another element is his decision - making. He knows when to take risks and when to play it safe. For instance, he won't force a pass into double coverage if it's not necessary.