Refers to the arithmetic operation of addition. Often used in elementary math contexts; 足し算 (たしざん) is more common in everyday speech.
In elementary school, we first learn addition and subtraction.
Multiplication, another basic arithmetic operation.
Division, another basic arithmetic operation.
Compound of 加え (kuwae, continuative form of 加える 'to add') + 算 (zan, 'calculation'). The kanji 加 means 'add' and 算 means 'calculate'.