Full Idea
To have a truth-value, a first-order formula needs an 'interpretation' (I) of its constants, and a 'valuation' (ν) of its variables. Something in the world is attached to the constants; objects are attached to variables.
Gist of Idea
A formula needs an 'interpretation' of its constants, and a 'valuation' of its variables
Source
Wilfrid Hodges (First-Order Logic [2001], 1.3)
Book Reference
'Blackwell Guide to Philosophical Logic', ed/tr. Goble,Lou [Blackwell 2001], p.13