The concept of Random is both relatively simple and also very difficult when dealt with definitively as in computer science. Random means lacking a definite plan, purpose, order or pattern. So for instance if a reference is made to random data, there is no specific meaning to that data as to its arrangements of 'bits' in order and no pattern that can be discerned on any scale which we can consider the data. Here we can get into the difficulties. If we look at a part of a data collection, we might perceive, or believe that we perceive, a specific meaning or order which at a larger scale, looking at more of the data collection, that pattern is seen to not continue and thus to not have existed. So the concept of random can be difficult to apply in practice.