The plus sign is a mathematical symbol used to represent a positive number or addition. This symbol is also endowed with rich abstract meanings because of its similarities with other things. The plus sign belongs to the first level operation.

Addition and subtraction are the two earliest mathematical operations mastered by human beings, and are contained in the earliest written records of human beings. The Amos cursive script in ancient Egypt contains a sign for addition and a sign for subtraction.

Source: @racheladawson