Migajas literally translates to "crumbs". It's an evocative word that paints a vivid picture of receiving very little.
In this song, Paty Cantú uses it metaphorically to express how she felt she only received "crumbs" of love from her partner, implying that she was given an insufficient and unsatisfying amount of affection and effort. This makes the word particularly poignant and memorable in the context of a breakup song.
The song opens as a list of everything the other person is about to lose. «Quién te abrazará de noche / Quién te besará al despertar» ("Who will hug you at night / Who will kiss you when you wake up"). Paty Cantú asks rather than accuses, and the questions keep reaching further ahead, until they arrive at old age: «Quién te seguirá queriendo / Cuando hayan arrugas en tu piel» ("Who will keep loving you / When there are wrinkles on your skin").
The chorus answers the title. Leaving is not the frightening part — «No tengo miedo a vivir si no voy de tu mano» ("I'm not afraid to live if I don't walk hand in hand with you"). What hurt was how little came back: «Sólo me diste migajas de ti» ("You only gave me crumbs of yourself"). By the last verse she has stopped wanting the thing she is losing at all — «Un pedazo de amor que no extraño» ("A piece of love I don't miss").