If you want to get a rocket off the Earth, you need to go faster than a special speed called the ‘escape velocity’ – the escape velocity depends on the gravity of a planet, which depends on the mass. If you don’t reach the escape velocity, you’ll make it part of the way off, but then crash back down again
Black holes were first theorised when someone thought ‘What would happen if the escape velocity was the speed of light?’ – this would mean you would need to go FASTER than the speed of light to escape it, which as we know, is impossible – nothing can travel faster than light.
A black hole is formed when a star above a certain mass goes supernova (collapses as it does); if a star with mass less than five times the mass of the Sun goes supernova, it collapses inwards then kind of bounces back and explodes outwards. If a star with more than five Sun-masses supernovas, the rate at which is collapses inwards is so fast and with so much energy it collapses inwards forever, and creates a black hole.
Because light cannot escape from a black hole, we cannot measure what is inside. The only way to find out would be to go inside, but then you wouldnt be able to send a message out again, so we may never know!
Because black holes can’t be measured, we had no way to confirm they were actually there – that is, until gravitational waves were discovered, which not only confirms that they are real, but opens up a whole new field of astronomy!