what is adding

Add

verb (used with object)

1.to unite or join so as to increase the number,quantity, size, or importance:

to add two cups of sugar; to add a postscript toher letter; to add insult to injury.

2.to find the sum of (often followed by up): 

Add this column of figures. Add up the grocerybills.

Adding is when you take two numbers so let's say 1 and 4, and you combined them together and you get 5