A mark is a status effect handed out by some PCs and monsters as part of an effort to encourage an opponent to attack the marker.
In dnd4e the condition "Marked" or giving someone a "mark" grants a -2 penalty to attack rolls on any attack that is not against the creature who does the marking. This serves the purpose of inciting attacks against the marker (often a defender).
Most PCs that are able to mark have a reprisal effect that triggers in addition to the penalty if the attack does not include them as the target.
In newer characters Defenders (for example the Knight and Cavalier from the Essentials line) now have auras that inflict similar penalties and reprisals instead of handing out marks.