Dreaming about a first love, especially in the context of them moving on with someone new, can evoke a range of emotions and may signify several things. Here are a few interpretations:

1. Nostalgia and Unresolved Feelings: This dream might reflect unresolved feelings or nostalgia for that relationship. It could indicate that you are still processing the emotions associated with that first love, even if you have moved on.
2. Fear of Loss: Seeing a former partner with someone new might trigger feelings of jealousy or fear of being replaced, which can symbolize a fear of losing connections or being unworthy of love.
3. Self-Reflection: This dream could also prompt you to reflect on your current relationship status. It may be a sign to evaluate what you want in your life now and whether you're happy with your current circumstances.
4. Personal Growth: Alternatively, this dream may signify your growth since that relationship. You might be recognizing how far you've come and how your experiences have shaped you.

Consider how you felt during the dream and your current life circumstances for a more personalized interpretation. If these emotions linger, it might be worthwhile to explore them further in your waking life.