I heard this for the first time today in shul. I was moved to tears (somewhat embarrassingly so, I might add.)
(The version they did in my shul was faster and was accompanied by two guitars, not piano, and it was less overtly shmultzy and therefore better in my opinion. But the above video clip is the best version I could find on the Internet)
The song is a riff on the verses in the Keddusha part of the Amidah prayer.
L'dor va-dor na-gid gad-le-cha,Below is a more traditional version of this prayer.
mi-pi-nu lo ya-mush l'o-lam va-ed,
ki Eil me-lech ga-dol v'ka-dosh a-ta.
Barukh ata adonai, ha'El hakadosh.
From generation to generation we shall relate your greatness
and for infinite eternities
we shall proclaim Your holiness.
Your praise, our God,
shall not leave our mouth forever and ever,
for You, are a great and holy king.
Blessed are Adonai, the holy God.