For a long time when people asked me what a project manager did, I told them I make the trains run on time.  That seemed to satisfy most people, but the more I think about it, the more I realize that it is beyond simplistic to the point of misrepresenting the job.

Sure, I make the trains run on time.  For very large projects, though, I usually have a project admin or ScrumMaster to help track whether tasks are completed on time.  That's a pretty small part of the job. 

Let's take the analogy a bit further, then. 

I  know what trains need to arrive where at what time.  I need to plot out routes so the trains don't run into each other and so that there aren't more than any train station can handle at once, and decide whether I need express runs that bypass some of the smaller towns to deliver people to their destinations earlier.

 I  balance the need to move the trains with the risk of trains waiting or even colliding.

I  advertise to get people to ride on the trains, hire the right number of conductors, drivers, mechanics, baggage handlers, cooks, etc. to staff the trains without overrunning my budget.

I  have the right number of trains available and move them to different lines as needed.

I  do ongoing accounting to be sure we are on track (so to speak).

I react when something goes wrong, anything from weather to labor strikes to trains colliding with something on the tracks, resolve the issue, and ensure that we work to avoid the situation in the future.  More than that, I anticipate potential problems and prepare for them - or figure out how to avoid them entirely.

I keep morale up, manage any disputes between team members, and build a cohesive, motivated team.  I also need to be sure they are  compensated and rewarded appropriately.

I determine what measurements I need to monitor to ensure a smooth operation and make corrections before any problem gets too big.

I report the status of the railroad and its risks to my stakeholders on a regular basis.

I  get help for my railroad team when something goes wrong or there's too much work for the number of people available.

So really, I don't just make the trains run on time.  I run the whole darn railroad.