Quid pro quo indicates a more-or-less equal exchange or substitution of goods or services. English speakers often use the term to mean "a favor for a favor". Quid pro quo is from the Latin meaning "this for that".
Additional meaning of Quid Pro Quo:
In legal usage, quid pro quo indicates that an item or a service has been traded in return for something of value, usually when the propriety or equity of the transaction is in question. For example, under the common law, a binding contract must involve consideration: that is, the exchange of something of value for something else of economic value. If the exchange appears excessively one sided, courts in some jurisdictions may question whether a quid pro quo did actually exist and the contract may be void by law.
The phrases with almost identical meaning include: "what for what", "give and take", "tit for tat", "this for that", and "you scratch my back, and I'll scratch yours".