You learn one additional spell at any level up to one lower than the highest level of spell you can currently cast. Thus, a 4th-level sorcerer (maximum spell level 2nd) gains a new 0-level or 1st-level spell known with which to expand her repertoire. For classes such as wizard that have more options for learning spells, Extra Spell is generally used to learn a specifi c spell that the character lacks access to and would be unable to research.