Whether you shoot on a cheap cellphone or a high end DSLR, whether film or digital, every camera essentially relies on 3 variables: aperture size, shutter speed, and ISO.
Shutter speed
A shutter traditionally blocks light from entering the camera until you're ready to take a photo. When you press the shutter release button on your camera to snap the photo, the shutter opens to let light in, and then snaps shut again. Shutter speed determines how quickly the shutter snaps open and closed.
A fast shutter speed lets in less light and "freezes" action.
A slow shutter speed lets in more light, but because the shutter is open for a longer period of time, this can result in blurriness - either because your subject is in motion, or because you are (hand-shake, breathing, body movement, ec).
Blurriness can be good or bad depending on whether you're intentionally using it to create an artistic effect.
You can use a tripod when you need to shoot on a slow shutter and want to minimize blurriness.
My camera was on a tripod to ensure the round lights appeared sharp. I used a 2 second shutter speed to create the ghosting effect. People who stayed (mostly) still during those 2 seconds look solid, and people who moved around during that time look ghostly.
My camera was on a tripod to ensure the tent stayed sharp, and used a slow shutter to capture as much light as possible because it was very dark, and I wanted to get enough light hitting the sensor to reveal the stars and Milky Way. You can also see a shooting star on the bottom right cutting across the Galactic Center of the Milky Way, and this appears because the shooting star was in motion while the shutter was open.
To summarize, here's a table of how shutter speed affects your photo-taking:
Please consider sharing your photos with the community by tagging #dharmaportraits on your Instagram photos - I'd love to see how you're using these tips to create beautiful photos.