Units and Constants:
Units are devices which scientists use to point out, estimate, and compute aspects of the world and the universe. Numbers by themselves are conceptual. Try to imagine the number 5 in your mind. You assume of a set or object: five objects, a line five meters long, five dots, a five-pointed star, or a pentagon. Though, these are sets or objects, not the real number. It is still more complicated to directly imagine the square root of two (21/2), pi (π), or the natural logarithm base (e), that aren't whole numbers.
Most of the people think of numbers as points on a line which are certain distances from the origin, or zero point. Displacement may be 21/2 units or π meters. You may think of a particular length of time, such as e seconds. May be you imagine of mass in kilograms or even something more exotic, like the intensity of an electric current in amperes, or the brightness of a light bulb in candelas.