Full Idea
When I say that a man exists, Frege argues, I do not predicate existence of a man, but rather of the concept man: I say the concept has at least one instance (and existence is a predicate of predicates).
Clarification
This is why 'existence is not a predicate'
Gist of Idea
'Existence' is not a predicate of 'man', but of the concept of man, saying it has at least one instance
Source
Roger Scruton (Modern Philosophy:introduction and survey [1994], 26.2)
Book Reference
Scruton,Roger: 'Modern Philosophy: introduction and survey' [Sinclair-Stevenson 1994], p.386