The basis of a properly exposed photograph is the concept called Exposure Triangle. This triangle is a combination of shutter speed, ISO and aperture. Before we get into that let us look at these three.
refers to the duration of time a camera shutter remains open to allow light to enter the camera sensor. This is measured in fractions of a second, typically for shutter speeds under a second. Slower shutter speeds are suitable for low-light and night photography as they permit more light into the camera sensor, while faster shutter speeds are ideal for freezing motion. Examples of shutter speeds include 1/15, 1/30, 1/60, 1/125 and so on.
is the hole within a lens that allows light to pass into the camera body. The larger the aperture, the more light enters the camera sensor. The aperture also controls the depth of field, which refers to the sharpness of the portion of a scene. A smaller aperture results in a greater depth of field, while a larger aperture results in a shallower depth of field. In photography, aperture is usually expressed in "f" numbers, which represents the ratio of the diameter of the lens aperture to the length of the lens. Examples of f-numbers include f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, and f/8.0.
is a method of brightening your photos when a longer shutter speed or a wider aperture is not feasible. It is expressed in numbers, with a lower number indicating a darker image and a higher number indicating a brighter image. However, increasing the ISO results in more visible graininess or noise in your images. Examples of ISO settings include 100, 200, 400, 800, and 1600.
To get the image properly exposed, so that it is not too bright or too dark, Shutter Speed, Aperture and ISO need to play together. When lots of light enters the lens (let’s say it is broad daylight with plenty of sunlight), what happens when the lens aperture/hole is very small? Lots of light gets blocked. This means that the camera sensor would need more time to collect the light. What needs to happen for the sensor to collect the right amount of light? That’s right, the shutter needs to stay open longer. So, with a very small lens aperture, we would need more time, i.e. longer shutter speed for the sensor to gather enough light to produce a properly exposed image.
To better understand this concept, here is a great video that explains this concept visually.