What is the meaning of "al-mustankah" in prayer?

"Al-Mustankah" (the person frequently assailed by doubt) is one for whom doubt has become frequent, occurring every day, even if only once. The ruling concerning him is the obligation to cast it aside, disregard it, turn away from it, and build upon the most probable. There is a difference between "Shakk al-Mustankah" (frequent doubt) and "Sahw al-Mustankah" (frequent forgetfulness). "Shakk al-Mustankah" is when doubt comes to him every day, even if only once, and he is not certain of anything. The ruling concerning him is to disregard it and build upon the most probable, with a recommended prostration of forgetfulness (sujood as-sahw). As for "Sahw al-Mustankah," it is when he forgets and it becomes clear to him that he has forgotten. The ruling concerning him is to rectify (the mistake) and not to perform sujood for it.