ISO is the how sensitive the light the sensor is. The lower the number the less sensitive it is, but also you get more detail. Therefore ISO 100 is "better" suited to daylight outside and higher ISOs such as 3200+ for dim inside.
To get front to back sharpness in landscapes, you need to consider hyperfocal distance and depth of field. or you simply dial in the smallest aperture you can (f22 for example) and focus 1/3 of the way into the scene. Keep the ISO as low as possible and use a tripod.
Learn the control of ISO, shutter speed and aperture for getting the correct exposure and getting the effect (movement, sharpness, bokeh, noise etc) you want, the learn the rest of the controls.