ISO tells your camera how bright or dark to create your photo based on the other exposure settings you pick. When we put them together, we call these three things the exposure triangle, but more on that later. Because the camera sensor picks up more light, ISO is referred to as the camera sensor's signal gain.
Base ISO, also known as native ISO, is the lowest setting your camera comes with out of the box. The native ISO range refers to the lowest to maximum ISO settings available out of the box on your camera.
Film sensitivity was used in the 35mm film era to describe how light-sensitive a strip of 35mm film was. A strip of film with a higher setting, such as 800, is more sensitive and burns faster when exposed. In film days, a film's speed was its ASA rating. For digital cameras, ASA was later adapted to ISO. A high ISO setting will inform your camera's sensor to absorb light more quickly. It is similar to how high-ASA film burns faster.
To acquire the correct exposure, you may need to increase the ISO on your camera to 800 or higher. On the other hand, raising it too high causes digital grain or "noise" in the image. This noise resembles static and, if severe enough, can render a photograph useless. However, most new DSLRs and mirrorless cameras perform admirably at high ISO settings. You can push them quite far without noticing noise.
When your camera is in manual mode, ISO interacts with two other settings to regulate the brightness and exposure of the photograph. They are shutter speed and aperture. The shutter speed is the length of time it takes for the shutter to open and expose the sensor to light. The lens aperture is the size of the hole your lens makes to admit that light in.
When your lens aperture is opened wide, it lets in a lot of bright light. If your camera's ISO level is too high, that light may blow out your photo. As a result, you'd lower the ISO to achieve an equal exposure. If you were shooting an outdoor portrait, you'd need to increase your ISO to accommodate for the additional light.
Various conditions will necessitate different camera settings. Changes in the environment or subject matter will all impact the ISO you should choose. A lit concert venue, for example, will necessitate a very different scene than a sunset on an open field.
You'll want to lower the ISO in brighter surroundings and increase it in darker ones. It's good to maintain the ISO as low as possible while still getting an even exposure to avoid digital noise. When it comes to measuring exposure, everything is a compromise. Achieving the right balance takes practice.
These are a few explanations to help you understand what camera ISO is and why it matters. We urge that you practice manual camera control as often as possible. When you no longer think about the fundamentals, your photography will take off.