Matter attracts matter via gravity. The more matter there is in the same place, the more this force is important. Our planet has such an important attraction that we are stuck on it, and it costs us a lot of fuel when we launch a rocket just to make him leave this attraction.
The stars are much more massive than our planet.
It has been imagined - by A. Einstein, I believe - that there could theoretically exist such massive and dense stars that their gravity could prevent even the light from leaving. These stars would be as massive as they were invisible, since no light could reach us.
These stars, we ended up noticing them, in particular by noting that the light coming from other stars was sometimes "displaced", and that because of the mass of a black hole being between this star and us, which because its gravity curves the path of this light.
A black hole, what is it?
Our star, the Sun, is not a fireball. Containing no air, there is not a single flame in it. It is a hydrogen ball. Originally a cloud of hydrogen whose atoms have gradually moved closer to each other by gravity, the Sun is now very dense because of the very intense force is caused by the gravity of so much matter. This density creates a significant gas pressure, which itself carries this gas at an intense temperature, causing it to melt. This fusion of hydrogen gives a slightly smaller mass of helium, the remaining mass being converted into radiation which evolves from the heart of the Sun towards its surface, mainly by convection, during a hundred million years, heating all that it crosses the way and increasing the pressure. The opposition of the gravity of all this mass and the pressure caused by the fusion generates what is called a hydrostatic balance: the gravity maintains the cohesion of the star while the pressure prevents it from collapsing on it even due to its gravity. When the pressure decreases, the gravity becomes too important and the balance is broken, the star collapses, in other words it contracts: its density increases and its volume decreases while its mass has not changed ( not significantly in any case). Gravity is altered because we now find so much material in a smaller space.
A black hole is a star with so much mass in such a small space that even a photon - the fastest particle known - can not escape its attraction, so light falls literally in, just like any other which mass passing too close.
The work in theoretical physics and astro-physics of the last century has demonstrated the very close relationship between mass and energy (which can each be changed in the other) and time and space (which are called space- time both are indissociable), mass / energy having the effect of bending space-time more or less depending on its quantity. Black holes are stars with huge densities, which means that they include so much material in a small space that their curvature of space time will be compared to a hole.
There are currently many hypotheses and many interesting works relating to black holes, but their nature and appearance are relatively simple to grasp.