Some types of surfaces reflect light. Reflection means that incoming light experiences a change in direction after having hit the reflective surface. Mirrors are a common example of reflective surfaces. This is distinct from refraction, which is the bending of a light's path when the light crosses into a different material (such as when light enters water or passes through window glass, etc.).
The incident ray is the original light before it hits the reflective surface. You can use a memory trick to remember this because incident light is incoming. The light is called the reflected ray after it has collided with the mirror and changed direction.
The angle of the incident ray and the reflected ray are both expressed with respect to a normal line drawn to the surface of the mirror. See the attached link for an illustration of this.