In a standard three-level defense, there are three layers: the forcing unit (consisting mainly of linemen but possibly also including blitzing linebackers and defensive backs), the underneath coverage (usually consisting of linebackers but possibly including other players, especially in the case of a zone blitz), and the contain unit (mostly defensive backs). A two-level defense does away with the contain unit in favor of increased pressure on the offense.
A three-level defense has three layers: the forcing unit (primarily linemen but potentially including blitzing linebackers and defensive backs), the underneath coverage (typically linebackers but possibly other players, especially in a zone blitz), and the contain unit (mostly defensive backs). A two-level defense eliminates the contain unit to increase pressure on the offense.