Top of Nikon FM2 controls

ASA/ISO dial shutter speed is on the oustide of the dial, it is set now to 125 which is 1/125 of a second which is how fast the shutter opens and shuts. On this camera it is highlighted in red, which means that speed is syncronized to the flash at that speed, so when using a flash with this camera you have to shoot at that speed to make it work properly with the flash attachment.
My FM2 uses 1/250 th of a second for flash giving it more capability to stop action with a flash shot.
My Pentax uses flash at 1/60 th of a second which limits it by not being able to stop action very well.

A rule to remember is if you have enough light to do so, use 125th or higher speed to stop action, to include movement from you the photographer!

The window inside the dial on the left is the film speed. Typically film can be purchased in 25, 64, 100, 200, 400, 1000, 1600, and 3200 ASA
These numbers are the film sensitivity to light. 25 ASA needs bright sunlight or powerful flash, but the grain is very small and produces the most beautiful photos you can imagine.
On the other end of the spectrum is 3200 ASA, it is very sensitive to light and can be used at night time and will produce photos in very low light but will be grainy and fuzzy at times.

I normally use 200ASA because it gives a good blend of low light and low grain, it is great for most things you photograph.

My father used 64 ASA Kodachrome slide film, his results were spectacular but again you have to use it in bright daylight or with a flash unit.