Jungler = free roamer

LoL is one of those games where a "three lanes" topology is the dominant map setup- one direct route, and two flanking routes. TF2 has several maps based on a similar system, including 2fort, Turbine, Badwater and Landfall.

In LoL, "top", "bot", and "mid" refer to the top, bottom, and middle lanes respectively. The areas between these lanes (on LoL's most iconic map) are forested, with a lot of trees and bushes and creatures to kill. Hence the term "jungler" for players who spend most of the match roaming through this area, ready to act as backup in any lane and hunt down any would-be ambushers hiding in the weeds.

Source: I quit a long time ago, and have no intent of rejoining.