From Wikipedia...
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Simple way to describe will be regardless the film size, if you shoot an object, for example a measuring tape or ruler, if you shoot at 1:1 the scale appeared on your film should be exact same scale as the ruler, 1cm=1cm, and if you shoot with a lens of 1:2, then the scale on the film will be 1/2 the scale of the object you shoot, 1cm=0.5cm. But of course, larger film will record more area but the scale is always constant.