There are 3 factors which relate to each other in controlling the exposure: the shutter speed which controls the time that the light has recorded on the sensor, the aperture which controls the amount of light recorded on the sensor, and the ISO which is the ability of the sensor to response to the light. Those three factors have the same criteria when changing for one step which is “one stop difference”. The meaning of one stop difference is the amount of light might be increased twice or reduced half depends on which direction it goes. For example, if you change the shutter speed from 1/30 sec to 1/60 sec, the time is shorter and it means that the amount of light will be reduced half, and if you change the f-stop from f/8 to f/11, the aperture will be smaller and it means the amount of light will be reduced half, and if you change the ISO from 100 to 200, it means your sensor is more sensitive and you will need just half amount of light of ISO 100. In vice versa, if you change from 1/60 sec to 1/30 sec, you are increasing the time and the amount of light will be increased twice, or if you change the f-stop from f/11 to f/8, the aperture will be bigger and the amount of light will be increased twice. Again, if you change the ISO from 200 to 100, you are slowing down the sensor speed and make it needs more light for twice amount of light.