The synecdoche of the title is both a figure of speech (a metaphor, where a part is taken to symbolise the whole of something, or vice versa) and a reference to the city of Schenectady, New York, where the action is set.
Writer and director Charlie Kaufman is a major movie-prodigy, and has cast a series of top-notch actors; Michelle Williams, Dianne Wiest, Emily Watson and late Philip Seymor Hoffman as a theatre director overtaken with his own life, which he turns into a play, and peoples with doppelgänger of himself and everyone around him.
If it sounds weird, it's because it is.
The screenplay is ambitious - I might even stretch that to 'pretentious', in fact - and somewhat reminiscent of Bunuel's 'Cet Obscure Objet du Désir' : It's the kind of film you could spend weeks analyzing and come up with heaps of interesting findings.
Problem is, I found it just Too Weird. Gave me a headache.