A long time ago I talked about three things involved in manual exposure. They are your aperture, shutter
speed and ISO, which stands for International Organization for Standardization. Trust me, I am not making this up.
Once upon a time film was sold (actually it still is) in varying speeds. For example you could buy 100, 200
or 400 speed film.
In digital photography ISO is the measurement of the sensitivity of the image sensor in your
camera instead of the film. The higher the ISO the less light required to take a photo. So in practical terms when
you're taking photos indoors you use a higher ISO than outdoors on a bright, sunny day. Your digital media, your
SD card for example, does not have a specific ISO speed the way film does. You insert any card in your camera
and then set the speed as allowed by the camera manufacturer.
If we go back to our room with the window covered by curtains, which represents your camera, you will get more
light in the room if you open the curtains at noon than if you open the curtains at midnight. At noon you could use
a low ISO speed and at midnight you would usea higher ISO speed.