The decimal $0.375$ can be expressed as the fraction $\frac{3}{8}$.

To convert a decimal into a fraction, it is essential to grasp the concept of place value. The decimal $0.375$ signifies $375$ thousandths, which can be represented as the fraction $\frac{375}{1000}$. However, this fraction is not in its simplest form. To simplify it, we must determine the greatest common divisor (GCD) of the numerator ($375$) and the denominator ($1000$).

Let’s begin by listing the factors of $375$: $1, 3, 5, 15, 25, 75, 125, 375.$ Next, we list the factors of $1000$: $1, 2, 4, 5, 8, 10, 20, 25, 40, 50, 100, 125, 200, 250, 500, 1000.$ From these lists, we can see that the highest common factor is $125$.

Now, we will divide both the numerator and the denominator by their GCD:

Thus, the decimal $0.375$ simplifies to the fraction $\frac{3}{8}$. This fraction is in its simplest form because the only common factor between $3$ and $8$ is $1$.

Understanding this conversion and simplification process equips you with the skills to convert any decimal into a fraction and simplify it effectively.