A non-batsman promoted up the order towards the end of a day's play with the idea of shielding a recognised batsman in the final overs
a batsman sent in to bat above his normal batting position at the fall of a wicket a short time before the end of a day's play, as a tactic to protect better batsmen from having to begin their innings just before the end of play and then have to begin again the next day.
In the sport of cricket, a nightwatchman is a lower-order batsman who comes in to bat higher up the order than usual near the end of the day's play. This batsman's job is to maintain most of the strike until the close of play (remaining in overnight, hence the name) and so protect other, more capable batsmen from being out cheaply in what may be a period of tiredness or in poor light. The theory is that losing two top-order batsmen in quick succession would be worse than losing one top-order batsman and a tailender.