It’s basically someone more experienced giving practical help and advice.

For a PPL student it could be flying with a PPL to see how things are done, perhaps going in to a larger field with full ATC, for example. A student could do the nav planning, check the NOTAMs, do W&B calcs for a trip. Then do the nav on the trip itself, without having the added stress of having to fly the aircraft. It could be to experience a cross-Channel flight with flight plans customs declarations etc. The time can’t be logged, but it is useful.

For a newly qualified PPL, it could involve having someone along to give advice and keep an eye on things or help with the workload. It helps to build confidence, but remember who is in command and responsible for the flight.

It is not teaching. That is for the formal flying instructor.