There are six types of quark (called "flavours") but only two "flavours" go together to make a pion. These flavours are called up and down. Quarks have charge, so two quarks of the same flavour (both up or both down) make a neutral pion. But when the two quarks have different flavours (up and down), the pion will have a charge. This charge is positive when an up quark pairs with a down antiquark. The charge is negative when a down quark pairs with an up antiquark.