It means, you have legal ownership and registration to use it on a public road.
Take VIN number and any old paper on it, with former owner signing it to you, to your local DMV or MVD, whomever. And some $.
Or, salvage title or apply for lost title and more $. Even more if lien on it.
(This is either plate and sticker good for a year or sticker only, and white and pink slip or Certificate of Title. Depending on the state.CA DMV; pink/white slip and tag (Plate, I don't remember.) .. AZ MVD; plate . tag, and C of Title. It must be notarized, another pain.)
(Oh Good, Chuckey, that reminds me: MVD double charged me and I have to call Phoenix to straighten it out. )