What is Correlation?
Correlation measures how closely two numeric variables move together, on a scale from -1 (opposite) through 0 (unrelated) to +1 (together).
Correlation explained
A high positive correlation between ad spend and sales means they tend to rise together. It does not show that one causes the other: both might be driven by season, or the link might be coincidence.
Correlation also captures only straight-line relationships and is easily distorted by outliers, so look at a scatter plot as well as the number.
Example
Ice cream sales and sunburn cases are correlated because both rise in hot weather, not because one causes the other.