1. Girls may want to express their gratitude to boys. In this case, salted duck eggs and crabs may be used as a symbol to represent girls' gratitude to boys.

2. Salted duck eggs and crabs may represent a prosperous fortune, which may imply that girls want boys to have a smooth and rich fortune.

3. Another possibility is that girls are expressing their concern and concern for boys. In this case, salted duck eggs and crabs may be a symbol, indicating that girls want to pay attention to boys' lives, or boys want to pay attention to their own lives and hope to establish a deeper relationship.

But these are only one possible explanation, and the specific meaning may need to be further analyzed according to the situation and the relationship between the two sides.