Troppo is, from a grammatical point of view, similar to molto, tutto, poco. In relationship to a noun in singular it describes the excess of something (measured in gram, kilo, etc.) and in relationship to a noun in plural it means that there are two many elements of something.
Beside that, troppo can be an adverb. As an adverb it it, obviously, invariable.
That's too pretty to be true.
Questo è troppo bello per essere vero.
I ate too much.
Ho mangiato troppo.
As well as in the indefinite adjectives / pronouns mentioned above, in some cases, even if used in singular, troppo can describe that there are too many elements of something.