The forgetting curve is the decreasing ability for the brain to retain information over the course of time. It is a very specific curve in which you can accurately see the rate at which a person forgets information. This curve was developed by Hermann Ebbinghaus (1850-1909) who used himself as a subject to develop the rate at which an individual forgets things.
Ebbinghaus developed a very clever and elegant empirical way to measure this. He memorized long lists of three letter nonsense 'words' like OBJ or KWT. He memorized these long lists and tested himself on his ability to recall the words. He used his rates of forgetting to form what became known as the forgetting curve. It is an exponential curve which means that the fastest loss of recall occurs during the first few days and tapers off after that. Afterwards forgetting still occurs but it has slowed down in comparison to the first few days.
The speed of the curve depends on factors like how stressful the initial learning conditions were and how rested the individual was while learning. The rapid loss of information during the first few days highlights the importance in refreshing learned material within that time so the information is less likely to be forgotten.