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