This is the phenomenon that two particles, however far apart, are deeply connected. It means that their states are dependent on each other, so that if one particle changes, the other particle changes accordingly, no matter where these two are located. So if particle A would point upwards, particle B would point downwards. Changing particle A to point downwards, immediately—like immediately, even faster than the speed of light—changes particle B to point upwards.
This allows to directly deduce the state of the entangled particle the moment the initial one is measured or its state is altered. We could use this to also send information instantaneously between those two locations.
Everything we know usually has a very distinct state. A table is standing at a specific position or a glass has water in it. We can describe it clearly.
Superposition, however, is the description of the state that a particle has, which is all states at once. The state is only humanly specified when measured. All particles are in a superposition until measured.
Imagine a flipped coin mid-air. We know it can land on either head or tail but while being mid-air, it has both states at once and neither of them for certain.
Regular bits—like we use in computers—are units to store information in. In computers, they store binary information, usually represented in 1 and 0. Qubits (short for “quantum bits”) are the computing units for quantum computers.
A qubit can store information in a superposition. This allows the qubit to store two bits at once. Reading the value of a qubit, however, forces it to decide for either state and thus also altering the superposition it had up to this point.
Particles behave like, well particles. But also they behave like waves. This is the crux of quantum physics. Especially the double-slit experiment displays this phenomenon. If one shoots light on a specific double-slit, then the lightwaves generate interferences, creating a pattern on the wall behind it, displaying not two slits but bigger and smaller stripes.
Doing the same experiment with non-wave elements—imagine shooting throwing a ball at equivalent slits—then the wall behind it shows two distinct lines, like the slits.
Particles, like electrons, behave like both. If not measured, they align in the interference pattern of waves. However, if measured, they create a clear two-slit pattern. Basically, this means that photons are in each position when approaching the wall—superposition—but once measured, they collapse into one state.