Twinkling is caused by masses of air of different temperatures passing between the observer and the object being observed. They have the same effect as poor quality window glass when trying to look through it. The image waves around and the atmosphere acts like an out of control lens that keeps changing the focus. To the unaided eye, stars twinkle or change brightness irratically. Through a telescope, twinkling stars dance around like a drop of water in a hot frying pan.
Planetary detail is smeared and fuzzy and little of interest can be seen on such a night.
In general, the higher up in altitude, the better the seeing because there is less atmosphere to see through. This is one reason professional observatories are usually located high up on mountain tops. In order to discuss seeing intelligently and to help in site selection of large observatories, seeing must be quantified. It is also important to know what the seeing is when evaluating a telescope . To quantify seeing, one could simply judge the amount and amplitude of twinkling but it would be to hard to get a good handle on the range or value.