Correlation is a measure of the connection between variables. For example the amount of leg room on a flight and the cost of the ticket. This video explains the concept well:
You can try this yourself. For example, what makes people happy? The OECD measures life satisfaction and publishes the data, here.
If we take three variables and measure correlation with life satisfaction this gives the following plot.
The darker the shade of grey the stronger the correlation. The top left to bottom right diagonal can be ignored, this is comparing the same fields so equals 1.
The strongest correlation is between satisfaction and earnings, to a lesser extent education and finally personal time. Displaying data visually is often easier to read than when the data is displayed as a table. For example the above plot suggests the strongest correlation is between life satisfaction and earnings.
The function to generate this plot is:
Function to visualise a correlation matrix