The abacus is a counting frame that has been used throughout history, first found around 2500 BC. Before we had a written numerical system, merchants would use an abacus to count a large inventory easily.
Over time, abaci have evolved into becoming our modern day calculator. However, the abacus itself does not do the calculation, it merely helps the user to keep track of amounts that they calculate in their head. An abacus is typically designed as a board that holds free sliding beads or discs on rods.
How to use an abacus:
Each row of beads, or in our case, rocks, represents something different. The bottom row representing ones, the middle being tens and the top row being hundreds. To start, all the rocks would be pushed over to the left side. For each number, you would move the corresponding number of pieces. For example, 34 would be 4 pieces moved on the bottom row, and 3 pieces moved in the tens row. Math can be easily done by adding or subtracting a number of pieces from each row.
For more clarification and a more in-depth explanation: