Event horizons are boundaries in spacetime beyond which nothing, including light, can escape the gravitational pull of a black hole or any other object with sufficient gravity. They are a consequence of the intense curvature of spacetime caused by the mass of the object.
Inside the event horizon, the gravitational force is so strong that the escape velocity exceeds the speed of light. As a result, any object or signal within the event horizon would need to move faster than the speed of light to escape, which is not possible according to our current understanding of physics.
Gravitational waves, however, are a different type of phenomenon. They are ripples in the fabric of spacetime itself, caused by the acceleration or movement of massive objects. Unlike particles or electromagnetic waves, gravitational waves are not subject to the same limitations imposed by the event horizon.
Gravitational waves can pass through the event horizon because they are a distortion of spacetime itself. They are generated outside the black hole's event horizon, by the motion of massive objects in its vicinity, and can propagate through space just like ripples in a pond. However, once inside the event horizon, the gravitational waves will ultimately be drawn toward the singularity at the center of the black hole, where our current understanding of physics breaks down.
So while nothing, including light, can escape from within the event horizon, gravitational waves are able to leave the vicinity of a black hole and travel through space. This is because they are not bound by the same restrictions as particles or electromagnetic radiation.