In response to the poem written by the shepherd to his love, the nymph is saying that his lovely idea of what life will be like together will quickly turn ugly.
The shepherd has promised his love all the beauty of nature...the waterfalls and flowers and glades and warmth of the sun, but the nymph reminds him that these things are fleeting and seasonal. Quickly the warmth of summer will be over, there will be no sheep in the fields and the meandering streams of summer will turn to cold raging rivers of winter and spring. Their love would be the same because it is based on his imagination and not reality.
