The Constant of Proportionality refers to the ratio of change of one variable that depends on another one. For example, we know that speed is a variable that depends on time, if we say, Sam traveled 18 meters per second, we are showing a ratio of the change of Sam's position because he moves 18 meters per second, this ratio of change is a constant of proportionality since it relates two variables.
Another example would be the number of Celsius the temperature changes if we say the temperature is changing at a rate of 3°C per minute, we are expressing a constant of proportionality since it's representing the ratio of change between two variables: Temperature and time.