As the magnetic field sweeps through the wire, it pushes the electrons in one direction (here, to the left). These electrons push on other electrons, starting a flow of electrical "current" through the "circuit."
It’s a "circuit" because you can?t just cram more electrons into a piece of wire. If you try, as the number of electrons exceeds the number of protons (in the atomic nuclei), the wire charges negative and pushes the extra electrons back out. Electron clouds are like water - you can?t compress them - they have to flow somewhere.
