Leaving snow out of it for the moment, most armies use two different camouflage patterns, or at least camouflage colours, which are optimised for certain areas. These tend to be at the opposite ends of the colour/darkness spectrum and are typically a woodland cam (dark, greeny/black colours) and a desert cam (light, sandy colours).
These cams tend to work very well against their target backgrounds, but their performance drops off as they move along the spectrum towards the end they are not designed for. By the time they are somewhere in the middle – say: grassland, arid scrub, urban areas – they are either too dark or too light to be ideal; and beyond a certain point they stand out like ticks on a dog’s balls: think of Woodland DPM against Salisbury Plain grassland or Desert DPM against a brick wall for example.
Multicam adopts a different approach. In effect it is optimised somewhere near the middle, and has colours and an overall brightness that means it never quite reaches the point of being utterly wank, no matter what background it is seen against. Put next to a terrain specific camouflage on its own turf it will never be quite as good; but it is never lethally bad either, and across the whole range of backgrounds it gives better overall performance: it is a multi-terrain camouflage.