Introduction to Summation Notation and Series
You'll need this to know a tiny bit about the next topic.
The summation, denoted by the uppercase greek sigma symbol,
Sometimes the sum has lower and upper limits, you'll see this as a certain variable from a value to another.
Take this for example:
Basically, take the expression that follows, do it for each value, then add them all up together. Easy.
This is a series. The sum of all factorials of 2n from n=1 to n=10.