Objects with large masses Warp Space in towards the object, but in three dimensions like in this picture. This warp is felt as gravity. The theory that describes this is general relativity.
A good way to think of this warp is like the lines of graph paper being bent in towards a mass (an object). A 2-D graph image, like this image is a little easier to view than the last image.
Gravity works by pulling other objects into the warp or bend in the "paper" of reality. This is how we get planets and moons orbiting other objects.
It's like a bowling ball called Earth is bending the paper.
So what's the problem? While both theories (General Relativity and Quantum mechanics) are good at describing their parts of the universe they end up disagreeing with each other and conflict with each other. Given the nature of, well nature you can't have to different and disagreeing sets of rules governing the universe
Remember General Relativity for a second. The bigger the mass the more bent the fabric of space gets and the more "gravity" an object has. Black holes are thought to be really giant masses condensed into a really small space. The result is an abyss of gravity that even light gets sucked into. Since this abyss is made by a really large mass in a really small space black holes will require General Relativity and Quantum Mechanics to be unified in order to be explained.
What is string theory? String theory is the idea that everything at its most fundamental level is made of vibrating strings. Depending on how these strings vibrate they act as different particles, such as the particles that act as gravity, light, the building blocks of neutrons and protons or anything else in the universe.
Is that all? While you get the general idea of the basics of modern physics here there is a lot more to it like how there are 11 dimensions of space not just the three dimensions you see when you watch a 3D movie. Since a lot of that can be a bit hard to understand let's spare your brain and you can continue to explore the topic on your own. Good Luck!