There are two aspects to frame rate (FPS).
There is the rate of playback – that is what is set with the FPS setting in Blender, the target playback rate, the rate at which you intend to play the animation when it’s done. Remember – Playback rate. This will also adjust the frames/seconds scale in the Timeline – FPS is Frames Per Second, how many frames will pass in one second when played at that frame rate.
For real-world cameras there is also the recording frame rate, which is the speed at which the camera actually records frames during the action it’s filming. Many cameras can do moderately fast frame rates, say up to 48FPS or maybe even up to 100 FPS. But it takes special engineering to get the rate way up there – they’ve even achieved rate of up to 1 million FPS or more, for scientific studies (like a-bomb explosions – cool as hell to see, not so good to be there). When recorded at a high frame rate, an action is slowed when played back (projector frame rate, or video frame rate) at a lower (slower) frame rate.
It’s pretty simple when you think about the actual physical situation of using real film – say a camera takes 2400 frames to record 1 second of of action (recording frame rate of 2400 FPS). A projector that plays it back at only 24 FPS will take 100 seconds to go through the action (2400 frames / 24 fps = 100 seconds), and that stretches the time out by 100X – a decent slow motion.
Blender’s virtual camera does not actually record existing action, it is used to create that action. So to get the same effect, you need to create the action spread out over many more frames than usual. So if you keyframe your action for 2400 frames and play it back at 24 fps it’ll look slow if that same action normally takes only 1 second (24 frames) to happen.
Here’s a little experiment – animate a ball to bounce for a couple of seconds, it doesn’t even have to be good animation, just 2 seconds worth of motion. Play it back with ALT+A. Then go into the Action editor, set the frame marker at the first frame of the action, select all the keyframes and scale them by 10 (key S, numpad 10, then Enter). Now play it back – 10X slower, because you now have 480 frames instead of 48 (if 24 fps was you chosen frame rate – it’s a common feature movie frame rate for historical reasons). More frames for an action, the slower it will appear at a given frame rate.