Heads is an option for functions which use level specifications that specifies whether heads of expressions should be included.
Heads -> True treats heads just like other elements of expressions for the purpose of levels.
Heads -> False never includes heads as part of any level of an expression.
Most functions which use level specifications have the default setting Heads -> False. One exception is Position, for which the default is Heads -> True.
See The Mathematica Book: Section 2.1.7.
See also: Level.