In gross simplification terms, imagine we have isolated a single electron that we can measure and manipulate. It is suspended in a medium or created in a superconducting loop.
That single electron is combined with a series of other isolated electrons.
Each electron and the parts that manipulate, isolate, and measure the quantum state of the electron is one qubit, or one quantum transistor.
While these individual quantum transistors are tiny, the hardware to do all the supercooling, measuring, and parts to make it up take up a lot of space.
A normal transistor can exist in only two states. On or off. 1 or 0. The power of qubits is that they can exist in any number of superpositions states of both at the same time, fundamentally changing how math can be performed as it searches through the varying states of 1 or 0 to solve a problem.
I think a weird way to picture how a quantum computer might work is from a marvel movie (with a giant leap of lateral thinking). Dr. Strange at the end of Infinity war is examining and searching through millions of timelines simultaneously to find an answer to stop Thanos. If he were to go and examine timeline by timeline one at a time he would never yield an answer fast enough. By existing in a weird state of psedo-magic superpositioning between all the timelines, he can come up with an answer faster.