Concealed was a new game mechanic that was introduced with Marvel Knights.
Concealed characters go into another part of the field apart from the normal field called the "Hidden Area" characters in the hidden area are exactly like characters in the visible area (normal character cards and ones that aren't concealed, etc) with one exception that they cannot be attacked (not even by other characters in the hidden area). If a player only controls hidden characters and no visible characters, then they're opponent may be able to attack directly, because there is no other legal target.