What does the sum of integers from 1 to n mean?

Natural numbers are positive numbers starting from 1. These can be written as:

$1,2,3,4,5,6,7,8,9,10......$

We want to write a program that takes a number and sums up all the numbers from 1 until that number.

Below is a visual to help you understand:

The sum of all numbers up to a number is equal to the sum of that number and the sum of all the numbers before it. It can be seen as follows:

$\sum_{i=1} ^{5} i$

$=$ $5$ $+$ $\sum_{i=1} ^{4} i$ ...