The fraction 3/8 is in simplest form. Reducing a fraction to simplest form means that each number in the numerator and denominator have no other common factors that you can divide the fraction further by.
For example, for the fraction 6/16, you can divide both numbers by 2, reducing it to 3/8. Fractions in simplest form are equivalent to the original fraction.
If you take a closer look you can obviously see that 3/8 IS in the simplest form. There's nothing you can do to make more simpler. If you want to get some decimals, simply divide 3 by 8 which will give you 0.375.