As well as ground meaning the venue for a match (e.g. 'the Sydney Cricket Ground'), the word has a very important meaning - especially to batsmen - in how the game is played.
A batsman is said to be within his ground if his bat - providing he is holding it - or some part of his person or equipment is grounded behind the line marking the popping crease. If he has 'made his ground' or is already within it when the wicket is put down with an appeal he should not be given out Run out or Stumped.
The decision as to which ground belongs to which batsman can create some difficulty - though more for commentators, it seems, than for umpires.