Broadly, floor leaders are the party's leadership in the House (and the Senate, too. They have floor leaders)
Since the majority party elects the Speaker, the floor leaders are the #2 person the majority party and the #1 person in the minority party. They act as chief political spokesperson for their party and run the handling of votes and the discipline of members. They are often assisted by "whips" who act as sheepdogs for the members.