Legarti comes from the verb legare, meaning "to tie" or "to bind". It's an interesting word choice in a love song because it suggests a strong, almost inseparable connection.
In the context of "Tua", Mina sings, "Legarti a me, amore" (To tie you to me, my love). This phrase beautifully conveys the desire for a deep, lasting bond, implying a love so powerful that it metaphorically binds two people together.