3:10-14 What exactly is John the Baptist teaching here?

John teaches here that repentance is not an abstract religious term or a matter of form and ceremony, but a radical change from a self-centred existence of greed and dishonesty to a practical expression of moral and ethical relationships with others (cp Isa 58:6-8; Lk 11:37-41; 19:8; 2Cor 8:14; Ga 6:10; 1Ti 6:17-19; Jas 2:14-16; 1Jn 3:16-19).

Justice and mercy must be practiced among all people - those who have more than they need are to impart to those who are without. This is not teaching however that Christians must give all their surplus to others regardless of how dishonest or idle they may be (cp 1Ti 5:8).

Any Christian who does not provide for his own family is worse than an unbeliever. Paul commanded the Thessalonian Church to stop supporting Christians who would not work (cp 2Th 3:6-15).