Here I have explained SR. In short: c = constant in all frames, that means light leaving an airplane leaves with c and is approaching you, on ground, with c, which is logically not to be explained in 1st instance without headache.. SR has solved this dilemma and you can read it, see above. c = constant is a main axiom in SR.
GR’s or in German ART main axiom is the equivalence of acceleration and gravitational attraction.
Based on all axioms GR was developed until 1916 and verified on occasion of the ecplise 1919. A star nearby the sun has changed its position by that value EInstein has predicted.
Imagine a closed elevator that is accelerating very long. So you can’t differenciate if you are pressed to bottom by acceleration ore gravitational force. Don’t think about such an elevator which is not available, but a Saturn 5 with an accelreation of some g can be also taken instead. Now imagine a light beam, or more practical a bullet passing the cabin. On entering and passing the cabin, the elevator is increasing its velocity. So if you track the clue of the bullet you will see a curved pass thru. Just like a bullet on earth is also falling 5 meters in the 1st second due to the gravitational effect.
Light is also attracted in presence of a gravitational field.
The light beam is deflected.
If you can agree on that, more or less the main issue is catched. Now to one main effect that is conlusive, compelling.
On the top of Mt. Everest the gravitation is slightly lower than on sea level, because the distance to the gravity center of earth is greater. o.k. But why is the clock running faster on Mt. Everest?
If anything falls it is increasing velocity. But light can’t change its speed, because nothing including light can be quicker than c. So something other has to change to solve this dilemma.
To solve this dilemma, time and distances are changing, so fulfil the axiom of c = constant. That is all needed to have a basic understanding of GR or ART.
How strong the effect is please ask wikipedia.
A Gedankenexperiment: If you are placed just outsie a BlackHole and one of your years corresponds to one billion years in the normal = flat universe, you would see really fireworks of novae and supernovae’s. But the probability is high, that you are hit by matter entering the BlackHole and also the radiation around you would be too high to survive.
BTW, SR and GR have strong effects on the GPS System. If this would not be taken into account, your car-navigator would be senseless giving false readings.
Einstein is also speaking of bended space and has found another explanation for gravity with that. But for layman like me, you need not to go in deeper.