You can perform songs as if they were a higher level.
A heightened song has a higher song level than normal (up to a maximum of 6th level). Unlike other metasong feats, Heighten song actually increases the effective level of the song that it modifies. All effects dependent on song level (such as saving throw DCs) are calculated according to the heightened level.
MP Increase: The heightened song costs MP equal to a song of its effective level. In addition, the perform check DC increases by its new effective level.