First of all - let's introduce a standard average:
standard_average_of(a, b, c) = (a + b + c) / 3.
We are summing our numbers, and dividing them by amount of numbers summed (imagine distributing what we collectively have equally among us).
But what if we need to equally distribute a set of numbers, where b is mentioned twice?
We could use a formula
(a + b + b + c) / 4, or
(a + 2b + c) / 4.
This is a
weighted_average_of(a, b, c), where b is weighted twice.