You haven't included the poem, nor enough information to find it online, but I can guess based on the context what the answer is. The words 'dream', 'lazy', and 'lullaby' likely give the poem its mood, or atmosphere. Poets often make use of connotation to affect how the reader views what they're saying. For instance, if you were describing a summer day, you might use words with different positive or negative connotations to affect the mood of your writing.
The sun shone down on the emerald grass, glowing against the azure background of the sky.
The sun glared down at the swampy green grass, burning against the pale blue, empty sky.
The first sentence contains words with positive connotations, such as 'shone' and 'glowing', while the second sentence contains words with negative connotations such as 'glared' and 'burned'. While both sentences mean the same thing, the word choice affects the mood of the writing, or the way the reader perceives it.
