The plus sign was created by German mathematician Wei Demei in15th century. Adding a vertical line to the horizontal line indicates an increase.

He also created the minus sign. Subtracting a vertical bar from the plus sign indicates a decrease.

The following is taken from the network.

The symbol of reduction is a symbol made by a horizontal line when the crew uses the water in the bucket to represent the water consumption of the day to indicate the reduction of water. The subsequent subtraction symbol is "-". ?

When the crew refilled the used bucket, they added a vertical line to the original "-"mark, so they added "+". ?

The symbols of-and+first appeared in German arithmetic books in 1489.