The Holy Spirit is a spiritual being [a non-physical entity] that is part of the Christian belief in the Trinity [the three parts or aspects of God]. It is believed to be the presence of God that provides guidance, comfort, and spiritual power to believers. For example, some Christians believe that the Holy Spirit helps them make important decisions or gives them strength in difficult times.
Very Simple Definition:
In Christianity, the Holy Spirit is a special part of God that helps and supports people who believe in Him. For example, some people think the Holy Spirit gives them strength when they are sad or helps them know what to do when they have to make a choice.