The 1911 is a single-action-only pistol. What that means is that the hammer has to be cocked (pulled back and locked in place) for it to fire. Pulling the trigger only performs a single action, namely releasing the hammer from its cocked position. Hence the name.
In a SAO pistol this must be done manually by the user, either by racking the slide, or physically pulling the hammer back until it locks back (its usually done when loading a magazine and dropping the slide). At that point pulling the trigger releases the hammer which contacts the firing pin and fires the gun. The action then slides back with gas pressure and recoil, racking the slide and automatically locking the hammer back for the next shot.
If the hammer isn't locked back, the trigger being pulled doesn't accomplish anything, since it's purpose is to release the lock holding back the hammer.
The alternative system would be a double-action/single-action. In that case (such as with a Beretta 92 / M9) pulling the trigger will both pull back and release the hammer. Note that the hammer-down uncocked trigger pull is heavier (more finger pressure is required) than the subsequent single-action pull.
There's also such a thing as a double-action-only, but those are mostly revolvers. There's the odd DAO pistol, but they're not terribly popular as the DA/SA system is generally seen as superior.
As an aside, a lot of people think it's unsafe to carry a pistol with the hammer locked, but for the 1911 that's the proper way to carry it, with the safety on. Otherwise you end up in exactly the situation shown in the screenshot.
Edited for clarity and a typo