Hello!
The phrase "on the plus side" is used to introduce a sentence containing a positive perspective. We often use it when we have just discussed the negative perspective of a topic to indicate that there is also something positive to see.
For example:
"It is supposed to rain all week. We can't play outside. On the plus side, the plants will get the water they have been needing."
Please let me know if you have any questions!
Savannah