You need two things to legally operate a vehicle on a public road in America: insurance and registration.
We cover insurance separately here. So what does vehicle registration mean in the US, and how is it done here?
A vehicle registration permits its operation on public roads
In many countries, ownership and registration documents are one and the same (v5c in the UK, the Rego in Australia, etc.).
However, in the United States, a vehicle's title establishes its owner, but does not, by itself, permit the vehicle to be operated on public roads.
A vehicle's registration is proof that:
The vehicle complies with all applicable inspection requirements (such as emissions, safety, and VIN verification), and
That all taxes applicable to the vehicle have been paid.