How much is a decade?
"Dec" is the Latin word for "Ten". In this case, "Decade" means "ten multiples" on the frequency axis. Example, one decade in frequency would be 10, 20, 30, 40, 50, 60, 70, 80, 90, 100. The range from 10 to 100 contains 10 multiples of 10. Another example would be 100, 200, 300, 400, 500, 600, 700, 800, 900, and 1000. The range from 100 to 1000 contains 10 multiples of 100. 

When the plot shows -20dB per decade, it means that over a range of 10 multiples in frequency, the gain will drop by 20 decibels.