Parallel summation is like adding a bunch of numbers together really fast with the help of many helpers. Imagine you have a task to add up 10 big numbers (let's say 456, 234, 567, 987, 123, 555, 789, 434, 888, and 222) and you want to do it quicker than you normally would.
To do parallel summation, you can divide the numbers into smaller groups and assign each group to a helper. So you might give two helpers the numbers 456, 234, and 567, and give another two helpers the numbers 987, 123, and 555, and so on. Each helper then adds up their set of numbers.
Once each helper has added up their numbers, you can combine their results to get the overall sum. So if the first two helpers got 1,257 as their total, and the second two helpers got 1,665, and so on, you can add those totals together to get the final answer, which in this case is 6,203.
By using parallel summation, you were able to add up all 10 big numbers much faster than if you tried to do it all by yourself.