Pennsylvania has a lot of rolling hills. Littler Round Top was one of the highest points in the area. Stationing your troops on "high ground" means that you have a better vantage point for a battle. In other words, you can see below and attack with ease. Picture standing on a chair and looking down at a room: you can see more, you can alert someone if something is coming, and you can aim better at whatever you're attacking.
Colonel Chamberlain was told to hold Little Round Top, a daunting task. The position was so important and militarily crucial, he was told to hold it until the very last man: people would die to keep this strategic location. He beat the Confederate troops to the top and dug in his heels, fighting to keep his position on this important rocky hill, and eventually the Union won the battle of Gettysburg. Without the high ground at Little Round Top, they very likely would have lost.