Going up one stop means you double the amount of light that hits your sensor or film. Going down one stop means you half the amount of light.
You can go up and down in stops in the 3 dimension you know from the exposure triangle:
Shutter speed: If you double the time the shutter is open, then you go up one stop. If you change the shutter speed from 1/100th of a second to 1/50th, is the same as going up one stop. You let in more light because the sensor is exposed for a longer time.
ISO: If you half the ISO setting, it is the same as going down one stop. Say you are shooting at ISO 1600, then reducing to ISO 800 is the same as going down one stop. Increasing to ISO 3200 is going one stop up.
Aperture: If you change the aperture from f/1.4 to f/2, then you are going up one stop. Same if you go from f/2 to f/2.8. When you walk up and down the aperture scale in hole stops, you walk in steps of 1.41 (the square root of 2). This is not as easy to remember as the linear relationship for shutter speed or the doubling when it comes to ISO, but you will quickly get used to it.
What is the point? It is to give you a common language for changes to the exposure, irrespective of which dimension in the exposure triangle is changed. So to maintain status quo, you can go one stop up in shutter speed and one down in say ISO, and get the same result as before. This is a smart way to have a common language to changes to exposure, without knowing what the specific camera settings are.
Related reading
What is exposure compensation?
What is a histogram in photography?
What is HDR? What is bracketing?
