What is the meaning of the term "cropped sensor"?
Basically you just record a part of the image, hence crop...
A full frame sensor is 36mmx24mm in size, a cropped sensor, in the case of Canon APS-C approximately 27,9mmx18,6mm.
Lenses are typically designed to throw an image circle with a diameter of 56mm to cover old wet film - or the equally sized full frame sensor. A crop sensor that sits the same distance away, covers a smaller area of the image circle and hence records only a portion of the image circle thrown - i.e. it crops the image thrown by the lens.
A "crop" sensor is a sensor (usually in a DLSR) that is smaller than the standard 35mm film size. 35mm sensors are referred to as "full frame". Images produced from a "crop" sensor are equivalent to the middle being cropped out of a "full frame" sensor image.
I don't really like either term - the 35mm frame size is wholly arbitrary, provided the viewfinder shows you the correct field of view then it doesn't matter that the sensor is smaller than 35mm - "full frame" sensors are in tern smaller than 645 medium format sensors, which are smaller than large format sensors etc.