A fraction like ¾ means 3 equal parts out of 4 equal parts. The denominator (4) is the total number of equal parts; the numerator (3) is how many are being counted. Each part must be the same size.
When you learned about halves, thirds, and fourths, you worked with the simplest fractions. Now you're building the full understanding of what any fraction means. A fraction is a way of describing a part of a whole — but only when the whole has been divided into equal parts. That word "equal" is not optional. A shape cut into four unequal pieces does not show fourths, even if there are four pieces. The parts must be the same size.
Every fraction has two numbers that work as a team. The denominator (the number on the bottom) tells you the total number of equal parts the whole has been divided into. The numerator (the number on top) tells you how many of those equal parts you are counting or shading. In ¾, the whole has been split into 4 equal pieces, and you are talking about 3 of them. In ⅝, the whole has been split into 8 equal pieces, and you are talking about 5 of them.
Think about sharing a pizza equally among 4 people. Each person gets one fourth (¼) of the pizza. If three of those four slices are gone, ¾ of the pizza is eaten. The denominator (4) describes the fair-sharing arrangement; the numerator (3) counts how many shares are accounted for. This connection to fair sharing is why equal parts are required — if the pieces aren't equal, calling them "fourths" is meaningless.
A subtle but important point: fractions can describe more than just shapes. You can talk about ¾ of a group of 8 pencils (that's 6 pencils), or ¾ of a cup of water, or ¾ of an hour. The denominator always names the size of the unit (one fourth), and the numerator counts how many of those units you have. This flexibility is what makes fractions powerful tools for measuring and describing the world.