。
If someone talks to you, answer right away.
Literal
Talked-to [passive-られ] [conditional-たら] immediately reply [object-を] do [soft imperative-なさい].
話しかけられたら combines passive (~られる) with the ~たら conditional: 'if you are spoken to.' The passive marks the subject as the receiver of the action. ~なさい marks this as a parental or teacher instruction.