(adjective)
not allowed to fly, as a punishment or due to some other factor, describing an aircraft or a pilot, or (informal) not allowed to engage in recreational or social activities as a punishment, describing a child;
e.g. All of the flights are grounded due to the approaching storm. The child is grounded due to her misbehavior.
(verb)
stopped an aircraft or pilot from flying, either as a punishment or due to some other reason;
e.g. The committee grounded the pilot due to the poor judgement he had shown.
(verb)
refused to allow one's child to go out and engage in recreational and/or social activities, typically as a punishment;
e.g. She grounded the children because they had refused to do their chores.
(verb)
gave someone/something a strong basis, for example provided someone with thorough instruction in something or provided something with a firm practical or fundamental foundation;
e.g. The professor grounded the students in the basic principles of the field. He grounded his arguments with several pieces of evidence.