If a councillor calls a point of order can the chairman just reply you are out of order

Question

Or must he explain why the councillor is out of order

Answer 1

Depends on whether the challenge on a point of order in the meeting is made in accordance with standing orders and perhaps the chair was stating that the point of order was incorrectly made but a good experienced chair would give the reasons why and allow the challenge to be re-made correctly.

Answer 2

The MSOs requires the chair to adjudicate on a point of order.  There has to be a discussion comprising what the PoO is (references if known / available) and then the chair should consider it before delivering a judgement.  Just to reply with "You're out of order" sounds like playground nonsense.