This means that every employee is expected to do the job of multiple people, and is expected to manage their time appropriately to be able to handle this. For me, I joked about working two shifts, and starting 'second shift' after the trading day ended and I handed off monitoring to Asia. I'd grab dinner and then start working on my coding assignments.
At the end of my tenure, it wasn't uncommon for me to work 15 hours a day and be on call 24/7.
Now, this isn't healthy by a long shot, but it is what's expected. You get used to it and you allocate your time to make sure you don't go crazy.
When I hear about these kids going through the ringer and working these 100 hour weeks, I'm almost incredulous because: a) this is the amount you're going to be expected to work in the future, but you need to figure out how to manage it and b) you've heard the stories, what did you think was going to happen ?
I have sympathy because it's hard, and I've been there, but that's the culture of the firm and if you can handle it, it's awesome. It's pretty inspiring to figure out what you're capable of under dire circumstances, you develop great skills quickly, and after GS everything else has the volume turned down.
Also, you can tap out and there's no shame in that.