A Sequence is a set of things (usually numbers) that are in order.
Each number in the sequence is called a term (or sometimes "element" or "member"), read Sequences and Series for more details.
In other words, we just add the same value each time ... infinitely.
(We use "n−1" because d is not used in the 1st term).
This sequence has a difference of 5 between each number.
Is that right? Check for yourself!
(called Sigma) means "sum up"
Footnote: Why Does the Formula Work?
Let's see why the formula works, because we get to use an interesting "trick" which is worth knowing.