Gumdrop has it. Got to be an official teaching on faith or morals; got to be in agreement with the totality of existing doctrine (hence agrees with the accepted Biblical truths); and finally, is normally delivered together with relevant authoritative bishops heading curial posts, rather than all by himself. So, for example a teaching on marriage that modifies or clarifies somewhat the current Church position about homosexual relationships would be cosigned by the members of the Congregation on the Family and probably members of other groups of Bishops who advise the Pope.
Papal infallibility is a totally false construct of the Catholic Church.