Escape velocity is the minimum speed needed for an object to "break free" from the gravitational attraction of a massive body.
Small Explanation on Escape Velocity
Escape velocity is the velocity at which the sum of an object's kinetic energy and its gravitational potential energy is equal to zero. At escape velocity the object will move away forever from the massive body, without additional acceleration (like propulsion) applied to the object. As the object moves away from the massive body, the object will continually slow and asymptotically approach zero speed as the object's distance approaches infinity.
For a spherically symmetric massive body such as a (non-rotating) star or planet, the escape velocity at a given distance is calculated by the formula below