Computing is the act of calculating something––adding it up, multiplying it, or doing more complex math functions to it. Computers are named for this process, because they can compute faster than most people.
The verb compute comes from a Latin word for pruning. You can think of it like cleaning up piles of data to get a clear result. If you run a shop and have lots of figures, for items sold and returned, and money paid for salaries and overhead, then you'll have to do some computing to know how much money you actually earned that day. It's also a type of engineering.