"Overextending" is the process of a player filling up his Monster Card Zones and/or Spell & Trap Card Zone with more cards than necessary, leaving the player at risk from cards such as "Mirror Force" or "Evilswarm Exciton Knight". Having a full field may also prevent them from playing new cards.
Cards exist to specifically counteract players when they overextend. "Mirror Force" is one such card, designed to work against players who commit too many attacking monsters to the field. Cards like "Heavy Storm" also exist for use against players who leave too many Spell and/or Trap Cards on the field.