What is the importance of "We the people" in the beginning of the US Constitution?

The importance is that for the first time in history, a national constitution explicitly stated that it had been drafted not by a monarch or a group of barons but by representatives of ordinary citizens. It expressed the fact that such government had been instituted by the democratic will of the people and not by the "divine right" of a monarch or ruler.

It is definitely a collective term; it puts forward the right of self-government of the national community. It does not express the ruling will of an individual person but the collective will of an entire nation.