What is a stop?
A stop is a doubling (or halving) of the light reaching the sensor. Going from f/4 to f/2.8 lets in twice as much light — that is one stop brighter. Halving the shutter time (1/250 → 1/125) is one stop brighter. Doubling the ISO (200 → 400) is one stop brighter.
Why crop factor matters
Camera sensors smaller than 35mm "full frame" capture a smaller portion of the image circle, which makes lenses appear longer. A 50mm lens on an APS-C body (1.5× crop) frames the same scene as a 75mm lens on full frame.