Whenever we see someone run across a field, jump over a fence, spread frosting on a cake, or cry from anguish, specialized cells in our brain called mirror neurons wake up and start calling out to the rest of our mind. Mirror neurons are collections of neural pathways placed throughout the brain that activate when someone experiences or views a certain action (Rizzolatti & Craghero, 2004). These mirror systems fire, imitating the observed action, leading to potential “mirrored” activation in equivalent areas in the brain.
Mirror neurons are the structures that enable our social world.
We use mirror neurons every day to internalize that lonely panda on YouTube, to feel happy when our friend gets an "A" on a test, and to try and decipher whether our significant other is mad at you, or just everything in general. Mirror neurons are special because they hook into the same systems we use to represent "self" emotions. This enhances our ability to understand each other, but makes us vulnerable to things that may confuse the signals caused by others, and the signals that come from our own volitions.
Note: The current argument does not support mirror neurons as a biomarker. Instead, they are simply an example of the ways that the human brain has learned to internalize observed behavior (Heyes, 2010).