It is the ratio of the area to the volume inside. Animals that live in polar regions have low ratios. They have short limbs, and large body mass so basically they look short and stubby. They have less skin exposed to the cold. Polar bears have low surface area to volume ratios. Compare that sort of body shape to a Dinka tribesman in Southern Sudan where it is very hot. In this region of the world, being very tall and very skinny keeps you cool. Dinkas have a high surface area to volume ration. The object commonly reputed to have the lowest ratio is a sphere.

basically, if you have cube and ball of the same volume, ball will have smaller surface. In fact, of all shapes of same volume, sphere will always have smallest surface. This is useful as surface is always the one that is most exposed to environment, so when these effects need to be minimized you need low surface-area-to-volume ratio, or a spherical body. If you need to increase surface to volume ratio, you would grind a sphere into powder, and while volume would remain the same, you would end with endless surface.