Is the one who guides you on the path (tareeq) in such a manner that you do not even feel the hardships of the journey (sulook) and reach the destination.
He sows and cultivates qualities in your heart in such a manner that eventually from within yourself there is an urge to do more good and abstain from sins. Even if a mistake is committed by chance the guilt, the remorse and the repentance that ensue are such that they aid your progress further.
The tip for optimum benefit is to keep in regular contact with him. Informing him about one’s condition explicitly, requesting advise and supplications.