"you have to push your lungs forward through the drag of the atmosphere."
That is a quote from Chris Hadfield's YouTube clip for his MasterClass.
Is there a list or description of what an astronaut has to do with their body during launch?
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A little expansion on what Digger commented. Assuming you weigh 200lbs, 3 gees will make it feel like you weigh 600lbs. You could imagine it as a 400lb person sitting on you. That's definitely uncomfortable, but it's only for 8-9 minutes. It's not dangerous if you're otherwise healthy.
Further, not only is it really not that many gees, but they're pulling in a different direction. Fighter pilots are sitting mostly upright, when they turn they feel the gees pull towards their feet, which also tries to pull all their blood away from their brain. Astronauts on the other hand, sit almost completely reclined. During launch they feel the gees pull towards their back; so their blood still stays distributed properly.
Edit: As Russell Borogove has pointed out, it's not even 3 gees for the whole flight. There's a brief window of about a minute near the end of the launch where the astronauts feel about 3 gees. Most of the flight is under 2 gees. Here's a NASA article on the subject; they use m/s^2, not gees in their data, but just divide their numbers by 9.81 to convert to gees. All of this just reinforces what I said above though; space launches aren't that strenuous.