Same Value, Different Numbers
12, 24, 36, and 48 all describe half of something. Cut a pizza into 2 slices and take 1, or cut it into 8 slices and take 4. Either way, you have eaten exactly half the pizza. That shared value is what makes a whole family of fractions "equivalent" to one another.
Mathematically, a/b and c/d are equivalent whenever a × d equals b × c. Every fraction has an infinite number of equivalent forms, because you can always multiply the numerator and denominator by any whole number without changing what the fraction is worth. Among all of those equivalent forms, exactly one is the simplest: the version where the numerator and denominator share no common factor besides 1.