Stray light is light that scatters randomly in the optical lens system. That means that not only the light coming from a specific point in the scene reaches a corresponding pixel on the image sensor, but also a mixture of light reflected from other targets.
Typical scenarios with stray light are high reflective targets close to the camera (e.g. white table). Therefore, the most effective way is to remove stray light causing objects from the environment.
In addition, there is SW filtering to mitigate this effect.