When you get along with someone, you're friendly or compatible with them. A babysitter might promise to take his charges out for ice cream if they can get along with each other for an hour.
Some brothers and sisters get along well, while others fight like cats and dogs. Countries that are allied tend to get along better than those that are in conflict — think of the Civil War, when the North and the South of the United States didn't get along. You can also use this phrase to mean "progress" or "manage to survive," so you might say, "I could never get along without pizza!"