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.