Complex sentence: The importance of the popping crease to the batsman is that it marks the limit of his safe territory for he can be stumped or run out (see Dismissals below) if the wicket is broken while he is "out of his ground".
Split into simpler sentences: The importance of the popping crease to the batsman is that it marks the limit of his safe territory. He can be dismissed stumped or run out (see Dismissals below) if the wicket is broken while he is "out of his ground".