Beloved of Ork Nobz in particular, choppas are usually immense axe-like weapons or brutal cleavers. Choppas frequently have a chainsaw edge to make them extra rippy when it comes to chopping through armour. In close combat choppas limit the saving throw an enemy model can have to a 4+ at best. So, for example, if a Space Marine in power armour or Terminator armour were hit and wounded by an Ork with a choppa they would have to roll a 4 or more to make their saving throw.
Errata — Chapter Approved: Codex: Orks Q&A (v1.0): The choppa's save-capping modifier applies only to Armour saves — it does not alter Invulnerable saves.