A vanguard is a (usually small) group of people who lead a movement. It comes from two old French words, avant (front) and garde (guard). In military terms, a vanguard is a group of soldiers who scout ahead to protect an army from surprise attacks.
In the political philosophy of Leninism "vanguard" refers to the "vanguard party". This would be a small group of dedicated revolutionaries who would lead the Communist movement in Russia.