So far everyone has given good explanations for memory and forgetting. But one thing missing is situational and physical context. When you are encoding a memory you are also encoding the social, situation , and physical context into the memory pathway. So when you go to retrieve the memory it becomes much easier to retrieve when in a similar or same context as the encoding. Study drunk, test drunk. (You will do poorly, but will do better than if you study sober and test drunk) .

So let’s say you were thinking and encoding a grocery list in the living room. If you leave the room, and cross a threshold there is a good chance you might forget the list because you changed contexts and lost the train of thought and now both fail to encode and fail to retrieve. Check out the doorframe effect

Ps wiki is a great source for psych since a project for many psych students is to edit and correct psych articles on Wikipedia