A stop is a doubling (or halving) in light; used to refer to all three elements of the exposure triangle: aperture, film / sensor sensitivity, shutter speed. So a bracket of three images at 1 stop difference implies a base image, an image with twice the exposure, and one with half the exposure. Those three images can then be combined while editing; the images with lower exposure provide details in bright regions (the clouds and sky), those with more exposure are blow out in those regions, but provide better details in the shadows.

Focus stacking is how both the foreground and background are in focus. In an image with the plant in focus, the mountains in the back will be out of focus. So you combine different focal positions to get an image in focus everywhere. Sometimes you use more than two, but for landscapes often just two is enough. (Macro work, with very shallow depth of field, often uses lots of images stacked.)

This is all done with the camera on a tripod, taken in rapid succession, so the light and subject don't change.