What exactly is space-time fabric? I have heard about space-time fabric but I can't understand its definition. Is space-time fabric a physical thing or did Einstein imagine it to assist him with general relativity?
'spacetime' is real, however the word 'fabric' is made so that anyone can 'visualise' the meaning of space-time
You cant touch or feel the fabric, but you can perceive it through the bending of light over it. When someone says that the spacetime is curved around a planet, the following statement holds true:

the time is slower near the surface of the planet relative to far away from the planet

the space is 'curved' towards the planet, so if you send a space ship to the planet, the ship will curve towards the planet, even of it is travelling on a straight path. So the straight path is not so very straight to an external observer even if it is straight for a observer going through the path


The word 'fabric' is used so that anyone can easily 'visualise' the spacetime
Our Universe has a large fabric of spacetime, with several heavy objects bending and curving it here and there.
The word 'spacetime' has also a non-gravity related meaning. Einstein stated that as the speed of an object increases in space, the time experienced by it decreases
that means that if an object travels at the speed of light through space, it will not travel through time at all. And if an is stationary in space, it will travell the fastest through time