An example of a constant is C, the speed of light, in Einstein's famous
E = M C ²C is a speed, so it has dimensions of distance per time. For example, it has a value of about 186,000 miles per second, or 300,000,000 meters per second.
A dimensionless constant like Pi is different. Pi is 3.14159... regardless of whether you are measuring in miles, or in meters, or in carob seeds.