Absolute zero is the lowest possible temperature in this universe. You can not get lower than absolute zero which is -273.15 Celsius or 0 Kelvin. To understand why this is you first need to know how temperature works. Temperature is just the movement of particles in an object. The faster an object is moving the higher the temperature. The slower the particles are moving the lower the temperature. Now if we look at heat, there is no limit to heat in our universe because, there is no limit on how fast particles can move. However, when looking at cold the slowest anything can move is 0. So when the particles in an object have completely stopped, that temperature is known as absolute zero. A lower temperature can not be achieved because nothing can go slower than 0.
Now scientists know that absolute zero is when the particles are completely stopped, but can the get an object to a temperature that low. So far they have not been able to bring anything to 0 K. The have reached extremely close, which is 400 picokelvin (a picokelvin is one billionth of a Kelvin). The reason that is the lowest they have achieved is this is that when ever they slow down the particles, some extra energy gets in. In fact some scientist believe that absolute zero is not achievable in our universe, and to get that low, they have to go out of this universe. Now the way they cool it down that much is laser cooling, which involves hitting a laser opposite to the way the particles are traveling, therefore physically slowing down the particles.
Now some interesting facts about absolute zero are that, when you take certain elements to temperature close to absolute zero, they start acting all weird and break the law of physics. When some item like helium are brought down to temperatures near absolute zero, the become something called super-fluids. Super-fluids have no viscosity and no friction. This means that a glass would not be able to hold it. Since there is no viscosity, the helium would go right trough the glass. Some other things become superconductors, which means they have no electric resistance. No resistance means no matter how far you travel electricity, it wouldn't lose any power.
Now those are some details about absolute zero. Scientist will continue to try to reach absolute zero, but it may take a long time.