The ratio of red paint to blue paint is 2:1. What does the ratio 2:1 mean?

This means the for every 2 units of red paint, there is 1 unit of blue paint

exg

in a mixture, mix a red and blue paint to get a type of purple that is the ratio of 2:1 (red:blue)

so exg if you wanted 3 gallons you would do 2 gallons of red+1 gallon of blue since 2+1=3

it could also mean quantity

there are 2 tiles painted red for every 1 tile painted blue resulting in 2 unit of red paint to 1 unit of blue