The red drops on the leaf vein of the Valley oak (Quercus lobata) are called galls. When the female Cynipid wasp (Andricus kingi) deposits her egg on the leaf, she also injects a chemical. The tree’s response is to create the gall, which keeps the wasp safe and with food until it reaches its adulthood and emerges from the gall.

https://www.mercurynews.com/2018/09/06/students-make-tiny-discoveries-in-pollinator-garden/