The terms ortho, para, meta are used in organic chemistry to indicate the position of the non-hydrogen substituents on a hydrocarbon ring or a benzene derivative. 

Ortho- describes a molecule  with substituents at the 1 and 2 positions on an aromatic compound. In other words, the substituent is adjacent or next to the primary carbon on the ring.

The symbol for ortho- is o- or 1,2-<name of compound>

Meta-

Meta- is used to describe a molecule  with substituents are at the 1 and 3 positions on an aromatic compound.

The symbol for meta- is m- or 1,3<name of compound>

Para-

Para- describes a molecule with substituents at the 1 and 4 positions on an aromatic compound. In other words, the substituent is directly opposite the primary carbon of the ring.

The symbol for para- is p- or 1,4-<name of compound>