The present is the time that is perceived directly, not as a recollection or a speculation. It is often considered as a hyperplane in space-time, often called now, but it may also be viewed as a duration (see specious present).
Modern physics has not yet been able to explain what we normally understand by 'now'. Furthermore, there is no demonstrable reason why time should move in any one particular direction. This has led to the conclusion that the idea of a present is illusory and does not reflect the true nature of reality. The notion of "now" may be better understood as an unrealistic concept that has evolved in humans and animals to give us an understanding of reality useful only to the extent necessary for survival. For example, if you say "now", the moment you say it, it is in the past.