How does a mirror pick up light from vast distances away? And, if that light can reach a mirror, why can’t we see that light?
Your eye has two functional parts: a lens (that is about ¼cm²) and a light-sensitive surface (the retina) that is covered in rod-cells that detect light. It takes about 10 photons arriving at about the same time (0.1sec) for a rod to react to the light and send a signal to the brain.
This means that, unless about 400 photons arrive from a star onto every square centimetre then there isn't enough light to see the star, even under ideal conditions. In practice, many more photons are needed for your brain to be able to interpret the signal from your eye as a point of light.
In contrast the mirror of a telescope is much larger. A small amateur telescope could have a mirror that has an area 1000 times larger than the lens of your eye. This means that a star can be 1000 times dimmer, but still be visible when you look at it in the telescope. Large, professional, telescopes have mirrors that are millions of times greater than the area of an eye.
Moreover a telescope can be fitted with a camera, and film (or digital sensors) that are both more sensitive to light than your retina and can integrate the light arriving over a long period of time. If you make a long exposure, of several minutes, or even hours, then even dimmer objects will become visible. This can increase the sensitivity by a factor of many thousand again.
The combination of the large light gathering area of the mirror, with the sensitivity and long exposures make possible to "see" things using the mirror of a telescope that can't be seen with the naked eye.