Take a look at all of the layers of rock in this picture.
Each layer of rock represents a specific interval of geologic time.
Over time, layers of rock and sediment are laid down one on top of the other.
The oldest rock layers (and therefore the oldest fossils) are always on the bottom, and the youngest are on top.
This is referred to as superposition (super = above).
So, within one set of rock layers, you can tell which rocks (and fossils!) are oldest and which are youngest by their relative positions.
Let's zoom in on the rock layers, and the fossils within them, to take a closer look.