To put this as simply as I can...
If you have two particles entangled so that one is 'spin up' and the other is 'spin down' - you dont know which one is which, each is in a superposition of up and down (50%up and 50%down).

The critical point here is that when quantum physics states that each particle is a mix of up and down, this isn't simply handwaving, it isn't scientists saying "well we don't know which is which so we will say each is a mix of up and down".

No- it really physically is in a superposition of up and down.

Only when you measure the spin of one of the particles does it become definitively spin up or spin down- measuring it causes the wavefunction to collapse into one or the other.

But here is where things get really weird... lets say you measure particle A and its wavefunction collapses to give you a spin up for particle A. Your measurement ALSO causes the wavefunction for particle B to collapse into a spin down state intantaneously- no matter how far away particle B is...

If particle B is across the room, or across the galaxy it still will collapse into spin down at the moment you measure particle A.

This is why Einstein had an issue with quantum mechanics, calling this 'spooky action at a distance'.

This cannot be used to transmit information faster than the speed of light- observers light years apart measuring particle A and B will measure a random sequence of up and down, only when they compare their results they will see that when A was UP B was DOWN.

The above explanation simplifies the physics greatly, and glosses over the meaninglessness of saying two events are "simultaneous" (Two events that appear simultaneous in one reference frame may not appear simultaneous in other reference frames). I am also sticking with the Copenhagen interpretation of quantum mechanics, and not getting into others like the 'Many Worlds' interpretation (I actually like Everett's Many Worlds interpretation but it only makes things even weirder philosophically, while being identical in every way we can actually measure) - I am mainly writing this for others that may be interested, I guess you will just read it as what you would like it to say...