The plus sign is written as a cross of horizontal and vertical lines:
+
The plus sign indicates addition operation of 2 numbers or expressions.
For example:
3 + 4
means 3 plus 4, which is the addition of 3 and 4, which is equal to 7.
The plus sign is located on the computer's keyboard near the backspace button. In order to write it, you should press shift and = buttons.