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It’s a bit complicated, especially to put into simple terms. Unified field theory tries to describe all fundamental forces in a single mathematical model. It combines the Theory of Relativity, which describes gravity on a cosmic scale, and quantum mechanics, which describes the behavior of subatomic particles like quarks. Scientists have found that forces are actually mediated by tiny, tiny particles. The proton and neutron were thought to be the smallest particles but are made of smaller particles called quarks. The strong force, which holds quarks together to form protons and neutrons, is exchanged by a particle called the gluon. The weak force, which was discovered because it’s responsible for some forms of radioactivity, is exchanged by the W and Z Boson particles. Electromagnetism is mediated by the photon. When two electrons with the same charge come close, photons in a way push them apart. Quantum gravity still hasn’t been resolved by unified field theory, but some scientists suggest it could be carried by a hypothetical particle called the graviton. We still haven’t seen or found any gravitons yet because some of these particles are incredibly small and short lived. Scientists have seen concrete evidence of most of these particles at particle colliders, which smashes particles together and observes what happens and comes out. The Large Hadron Collider discovered another subatomic particle called the Higgs Boson. This is just scratching the surface too unfortunately.
If you would like a more in-depth and nuanced explanation, I would recommend this video: [The Standard Model of Particle Physics: A Triumph of Science](https://youtu.be/Unl1jXFnzgo).
The universe is ruled in different scales.
At a huge scale level, gravity collapses into a blackhole of which we don’t know much about what happens inside at those astronomical levels but we can infer some rules about how it works. Also what’s the shape of our universe, and that’s it, we don’t know whats bigger than our universe scale.
On a smaller scale, inside electrons are smaller particles called quarks and at that small level of scale other quantum rules apply.
At our scale, we have a general understanding of how gravity affects at our scale, we can measure time, galaxies, matter, space, we know how these things interact with each other and in general we have the rules on how these things work. How energy is converted, mass, and a bunch of other equations that explain the rules of our perceivable universe.
However, when we try to apply any of these rules to other scales, they just don’t work. At quantum levels other forces and phenomena happen that can’t be explained by “our scale” rules. Neither what really happens outside of our known universe.
A unified field theory would be a theory that would explain how all those scales are interconnected. So that we can plug a formula from “our scale” and get a deterministic corresponding value (or effect) at another scale.
“Unified field theory” is a slightly vague term that has been applied to various established and proposed theories. You’re presumably talking about one of three things. “Quantum field theory” is an approach to quantum mechanics that was developed from roughly the 1920s to the 1970s and turned out to be an extremely good way of describing particle physics. Within this framework, electromagnetism and the weak nuclear force have been pretty much fully understood. However, the strong nuclear force, which is the main thing that holds atoms together, is much more complicated. A “grand unified theory” would fully describe the strong force in combination with those other two forces. A “theory of everything” would also include gravity.
The main reason for wanting to develop theories that include all these forces is that there are processes that involve all these forces and presumably cannot be fully understood by considering them separately.
Also, historically, it has generally been very fruitful for physicists to combine disparate observations into unified theories. For example, in the 19th century people kept developing all kinds of new ideas about electricity, magnetism, and what we now know as electromagnetic radiation (light, radio waves, etc.). Various connections between these ideas were noticed, and eventually a unified theory of classical electromagnetism was developed that united them all and improved our understanding of all of them individually.
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