What does, "killed two birds in one stone" mean?

Read the following lines from wordsworth's poem and answer the question. for oft, when on my couch i lie in vacant or in pensive mood, they flash upon that inward eye which is the best paraphrase of these lines? despite being able to plant my own daffodils, i prefer remembering the ones by the bay. if i feel very sad, i look inside myself for courage to face the day with my eyes open. oftentimes, i become so tired and overwhelmed that i lay down and think of daffodils. when i am feeling empty or concerned, the sight of the daffodils comes to my mind.