The short answer is that external light bounces off of it to our eyes. If it wasn’t visible due to external light, then we could see it in the dark too.
Most, not all, of the cases where an object is emitting its own light, it’s because of temperature. Loosely, hot things emit light and cold things absorb light. Even light bulbs have some small hot filament or gas or something. There are other, more rare ways to get light from objects too.
When light strikes an object from the outside, it can be transmitted (passes right through), reflected, or absorbed. This is one dynamic for most objects.
The other is the object emitting its own light. If it is cooler than it’s surroundings it will absorb more than emitted. If it is warmer, it will emit more than absorbed. This is unnoticeable for many types of objects and temperature differences, but we can see some stuff glow when hot.
Things that absorb the most light (dark, opaque, not shiny things) are the same ones that emit the most light when hot. The most extreme case is called a black-body by physicists: it doesn’t reflect or pass-through at all. The whole deal is absorb or emit. So if it is cooler than the surroundings, it will be dark black with no light reflecting or being emitted. It will glow the most when hot. (They like it because can isolate and analyze absorb/emit dynamics as a function of temperature). This is how they figured out that the amount of light emitted is a function of temperature to the fourth power.
Emitted light = $k \cdot T^4$, where $k$ is a constant.
