There are many definitions of situational awareness, a simple one is, knowing were you have been, where you are and where you are going! I also like this one from the FAA but it should be remembered that SA is not just about geographic position it is also concerned with what is happening to the aircraft and its systems too.
Situational awareness is the accurate perception and understanding of all the factors and conditions within the four fundamental risk elements (pilot, aircraft, environment, and type of operation) that affect safety before, during, and after the flight. Thus, loss of situational awareness results in a pilot not knowing where he or she is, an inability to recognize deteriorating circumstances, and the misjudgment of the rate of deterioration. (FAA)
Situational awareness is mainly about staying ahead of the aircraft, never let the aircraft go anywhere you haven’t already planned for! SA starts on the ground with comprehensive pre flight planning. You should already however be very familiar with the aircraft and its systems before you even arrive for briefing.
PISS POOR PLANNING PRODUCES PISS POOR FLIGHT!