Simply put - although we can predict how most things behave pretty accurately we can never predict the behaviour exactly - there will always be a small degree of uncertainty.
On its own this is usually insignificant but events don't usually happen in isolation - they interact with other events which have their own small degree of uncertainity.
So as we look at larger and larger systems encompassing more & more elements, it becomes increasing difficult to predict accurately what will happen. Moreover the accumalative effect of all these small 'changes' can manifest themselves as large effects in the overall system. The example that is usually quoted
to demonstrate chaos theory is the infamous butterfly flapping its wings and causing (contributing to ?) a hurricane on the other side of the world.