Radiation pressure refers to pressure exerted by electromagnetic radiation. Electromagnetic radiation has a momentum associated with the energy it carries, i.e., the EMR's energy divided by its speed (the speed of light, c). Pressure on an object results when it absorbs, reflects, or emits radiation, the amount being such that the total momentum is conserved, the change in the object's momentum balanced by that of the incoming and/or outgoing EMR.
The pressure is small enough to be negligible in everyday life, but can be significant when it is steady over long intervals, e.g., for interplanetary navigation, which requires it as a factor. It is a factor in planetary migration, which are subject to pressures continuously for millions of years.
When it is extreme, such as inside or at the surface of a star, it is also potentially a significant factor.