Cosmic rays are energetic, subatomic particles from outside the Earth's atmosphere. Upon impact with the Earth's atmosphere, cosmic rays can produce shower of secondary particles that sometimes reach the surface. A significant fraction of primary cosmic rays originate from the supernovae of massive stars. Others are produced by ordinary stars like the Sun. For example, during a solar flare many such particles are ejected from the Sun.
Most cosmic rays are atomic nuclei: primarily hydrogen, some helium, and a few heavier elements. Travelling through the Earth's atmosphere, they interact with the air and produce cascades of particles like neutrinos, pions and muons. Many of these muons arrive at the Earth's surface, and also at the MicroBooNE detector! There they produce tracks and it can happen that some of them are misidentified as neutrino events. Scientists have to work very hard to develop techniques that allow the rejection of cosmic particles!