Looks like spittlebug froth. They make bubbles with their excretions of excess sap mixed with air, technically not spit, while feeding off the stems of plants. They're nymphs (juveniles) of leafhoppers,.sometimes called froghoppers. Yup, it's basically bubbly pee/poop. It protects them from predators and acts as insulation. Due to the sap, it can also be quite sticky. And - I was an adventurous and stupid child - it's often quite bitter tasting.