Entanglement "simply" means that the other party will have a correlation with your measurements.
Without receiving information of what you measured (inevitably in a slower-than-light way), none of their observations can tell whether you did that thing or not. The behavior of a single entangled particle is perfectly indistinguishable from a non-entangled particle, it's just that for entangled particles certain combined results are impossible or unlikely.
Imagine being given a magic pair of dice which always adds up to 7 when thrown at the same time. If you take them to different rooms and get a 6, you know the other party should have gotten a 1, but you can't use it to transfer any information to them because no matter what you do, in isolation their results are indistinguishable from normal dice.
Without receiving information of what you measured (inevitably in a slower-than-light way), none of their observations can tell whether you did that thing or not. The behavior of a single entangled particle is perfectly indistinguishable from a non-entangled particle, it's just that for entangled particles certain combined results are impossible or unlikely.
Imagine being given a magic pair of dice which always adds up to 7 when thrown at the same time. If you take them to different rooms and get a 6, you know the other party should have gotten a 1, but you can't use it to transfer any information to them because no matter what you do, in isolation their results are indistinguishable from normal dice.