I would take it to mean "play on" or "step up." However, you must consider the way that it is used in the other verses, as well, to get a better picture of how it should be interpreted. It is not just in the context of a game, but in the context of life itself.
These words, according to Coulson Kernahan, author of Six Famous
Living Poets, were a call to all men and women to defend their country
in an hour of need. These famous lines demand that “in life’s
battle-field [sic], whether a battle-field only figuratively, or a
battle-field in reality to play the game.”
So when I use those two phrases above, it is more a matter of persevering in the face of the present challenges being faced, than it is simply a call to start up the game again. The first verse describes a very tense situation in the cricket match. The game is drawing to a close. The penultimate batter was just caught out. The captain is encouraging his (final) batsman as he sends him in to make the last ten runs needed to win the match. That encouragement that will stay with the man throughout his life, to spur him on in the middle of a dire situation in WWI, and it's the exhortation he will leave to the generations to follow him as his life ends.