In more detail
3/8 = 0.375. A proper fraction has a numerator smaller than its denominator; an improper one such as 7/4 can also be written as the mixed number 1¾. To add fractions, find a common denominator with the Least common multiple (LCM); to simplify, divide top and bottom by their Greatest common factor (GCF). To multiply, multiply the tops and the bottoms; to divide, multiply by the reciprocal of the second fraction.
Why it matters
Fractions are exact where decimals often are not: 1/3 has no finite decimal form, and computers store 0.1 in binary only approximately, which is why 0.1 + 0.2 prints 0.30000000000000004 in most languages. Recipes, imperial measurements and probabilities are naturally fractional, and adding tops and bottoms directly (1/4 + 1/6 = 2/10) is the classic mistake.
Example
1/4 + 1/6 = 3/12 + 2/12 = 5/12 (LCM of 4 and 6 is 12)
2/3 × 3/4 = 6/12 = 1/2
2/3 ÷ 4/5 = 2/3 × 5/4 = 10/12 = 5/6
12/18 = 2/3 (divide by GCF 6)
Python's fractions.Fraction(1, 4) + Fraction(1, 6) returns Fraction(5, 12) with no rounding at all.
Common confusion
A fraction compares a part with the whole; a Ratio compares two parts. A drink mixed 1:3 is 1/4 syrup, not 1/3.