The subject acts upon itself. The direct or the indirect object is the same as the subject.
Non reflexive Reflexive Yo lavo los platos. Me lavo las manos. I wash the dishes. Me las lavo. Yo los lavo I wash my hands.

Mi madre se compró un almuerzo. Tú le compraste un almuerzo a ella.
Mi madre se lo compró. Tú se lo compraste. Mi other bought lunch for herself. You bought it for her.