This is a term distinguishing the essential elements of a contract.
To be valid, a contract must have three main ingredients:
Consideration (money or value).
A shop window displaying a property to let, is not, as it might appear, an ‘offer’. This is merely an ‘invitation to treat’. i.e. passers-by are being invited to come into a letting agents office to show an interest in any number of properties, perhaps view some, and ultimately to offer to rent a specific property.
Even after accepting an offer to rent this is merely an agreement which is not yet legally binding – not until consideration (payment) is also part of the agreement (absent vitiating factors) is there a legally binding contract. Vitiating factors might include something which induced the deal e.g. by misrepresentation. E.g. “You told me you were 18, but I subsequently I learned that you are only 14, therefore, you do not have the legal capacity of an adult to enter into such an agreement. This said, agreements of necessity, made by say a 16 year old can be legally binding as housing is necessity, unlike say buying a car, which is not likely to be.