The way any given society is organized; its internal hierarchy. Individuals are placed on a vertical rule, or «ladder»; Their position on this ladder (high being «good» and low being «bad») is determined by their level of attainment of a certain quality perceived as «desirable» by the community as a whole.
1) in a modern sub-society such as highschool, "physical strength" and "possession of expensive clothing" are considered «desirable» qualities; thus, individuals possessing these qualities are positioned high on the ladder («good»), while individuals possessing none of these qualities or their opposites are positioned low on the ladder («bad»).
social ladder – n. Metaphorical ladder that nonlosers are continuously trying to climb.
analogy The social ladder is like Mount Everest: Lots of people try to climb (be popular), but few reach the top (become popular). At any point you can fall into a crevice (fall off the ladder), or fall victim to hypothermia, starvation, fratricide, etc. (drugs, alcohol, fratricide, etc.) and die (die). Also, getting to the top is completely pointless, unless you really want your name to be known. Except that nobody remembers who climbed Mount Everest anyway.
see my definition of loser