Mirror neurons were first discovered in monkeys but the relevant brain activity has also been found in the premotor cortex, the somatosensory cortex and the other areas in humans. As is well known, mirror neurons are active when one performs a specific movement, and when one observes someone else doing the same motion. Neurons in our brain “mirror” the action of the other, as if the observer himself were acting in the same way.
Giacomo Rizzolatti, one of the discoverers of the mirror neuron, says that the mirror neurons "are primarily involved in the understanding of the meaning of 'motor events', i.e. of the actions performed by others". That is, the monkey "sees the experimenter shaping his hand into a precision grip and moving it towards the food, it immediately perceives the meaning of these 'motor events' and interprets them in terms of an intentional act".
[Rizzolatti, G. and Sinigaglia, C. (2008) Mirrors in the Brain. Oxford University Press. p.97-98]
Thus, the primary function of mirror neurons is to perceive another person’s movement as an intentional action. In other words, the other person’s movement provokes the same potential movement, the same potential action, the same intention in us, through mirror neurons. In the fundamental level, we do not need to infer the other’s intention from the objective point of view, nor to project our own intention to another from the subjective point of view. (Here is the point to remember the theory of mind debate).
The other’s movement appears to us as a meaningful action, from the very beginning.