Entanglement is a a very simple property any lay person can understand that at the same time is completely baffling physicists to the point that Einstein, the one that first identified it (with Podolsky and Rosen in a famous article in 1935) used it to show that since it was a consequence of quantum mechanics by being absurd "proved" the inconsistency of quantum mechanics! Unfortunately, experiments over experiments proved it to be true and we are still today struggling with something that is against the Einstein Relativity (that has been proven accurate through experiments) and in synch with quantum mechanics (that has been proven accurate through experiment).
Imagine a coin. When you flip it you can get head or tail. If you get head, the other side is tail. If you get tail, the other side is tail. You got tail and you flip it once? Instantaneously you get head. It is a no brainer.
The problem is in the "instantaneously". According to relativity theory you can't exceed speed of light (not because light is fast but because that is the property of the space time continuum we live in, light is simply part of thatspace-time continuum and not having mass "has" to travel at the speed of the space-time). And yet experiments have shown that it is possible to create pairs of entangled photons so that if you flip the spin of one you automatically flit the spin of the other (like a coin....) independently of their spatial separation.
Now MIT and Belgrade University scientists have shown a way to entangle 3,000 atoms with a photon, that is, to change instantaneously some characteristics of those 3,000 atoms using a single photon. So far this is the largest number of entities, particles, simultaneously entangles (the previous one was 100 atoms).
Atomic clocks are based on the measuring of atoms oscillation made through a laser. The more atoms you have the better the precision. Actually the clock precision increases with the square root of the number of atoms. If, however, you use entangled atoms (that oscillate in synchronicity because of the entanglement) the precision increases linearly with the increasing number of atoms. Hence if you have nine atoms rather than one you get a precision that is three times better but if those nine atoms are entangled the precision increases nine fold.
This is nice, although given the precision achieved by today's atomic clocks (one hour imprecision for a clock that started at the beginning of time, some 13+ billion years ago) one may say that further increase in clock precision is not really a need.
More interesting is that using entangled atoms makes possibile to create very sensitive sensors to detect minute magnetic fields variation opening up better ways to search for ores and oil. Additionally, atoms entanglement can give a boost to quantum communications, a new area that would support secure communications.
We are really at the edges of science and technology, a grey area where understanding is hard and experiments creates big question marks and, at the same time, stimulate practical applications.