Light sensitivity, also known as ISO level, is one of the three pillars of photography, alongside wide aperture and shutter speed. It represents how sensitive to light the image recording medium is (like the photographic film in analog cameras), but it is a somewhat complicated concept.
A higher ISO value translates into more light absorption, which can be quite helpful when shooting in low light conditions, or when capturing fast-moving subjects. On the flip side, increased light sensitivity is often associated with image grain (or noise). For this reason, you may first want to try working with a lower ISO, adjusting the shutter speed and wide aperture accordingly.
An increase in each of these pillars has pros and cons; as an example, higher shutter speed may create motion blur. The ideal situation is to find a balance between ISO, aperture, and shutter speed, considering the type of setting, subjects, light conditions, and overall aesthetic goals.
Digital photography ISO levels may range from 50 to 6,400 (or much higher). Bear in mind that opting for low ISO means that more light is required, being, therefore, generally not recommended for dimly lit environments unless for creative purposes. In an optimal lighting situation, however, a lower ISO rating may translate into more color accuracy and appealing visuals. For each twofold ISO increase (let's say, from 100 to 200), half as much light is needed for the same level of exposure, as long as there's no change in aperture.
Most current digital cameras and flagship phones have an auto ISO function, which can be extremely helpful, especially if you're new to photography. It's sometimes possible to limit the maximum auto ISO value, in which case you'd be sure that the light sensitivity would not be increased to a level that you're not comfortable working with.