Deleveling means falling back to a lower level by losing EXP. It can be accidental or intentional (by dying over and over again until you lose enough levels to reach some target, lower level). About 25 deaths are required to lose enough EXP to down-level. This number ranges from 18 up to 33. Since you do not lose experience points at level 5 or below, the lowest level you can delevel to is 5.