As previously explained, the heart if made up of virtues, emotions, and various other elements. This spell allows you to take some virtue from a person who has it an abundance, and store it in the Locket. Just be sure to ask for permission before you proceed. Remember: a heart belongs to one person, and one person alone.
Take Heart is an everyday spell that allows the caster to take a piece of someone's heart and to be kept in the locket for later use. It is one of two spells Oliver gets after defeating the Guardian of the Woods, the other spell being Give Heart.