A round face has length and width that are close to equal, with soft curves where other shapes have corners. The cheeks are the fullest part, the jaw is rounded rather than angled, and the hairline tends to curve too. The overall impression is of a circle rather than a rectangle.
Round is not the same as heavy, and the two get confused constantly. Face shape describes the outline created by bone and soft tissue at the forehead, cheekbones and jaw. People at very different weights have round faces, and losing or gaining weight changes fullness without changing the underlying proportion much.
How to check whether this is your shape
Measure hairline to chin, then the width across the cheekbones. If those two numbers are within about ten percent of each other, the face is reading round or square rather than long.
Now look at the jaw. Run a finger from below the ear down to the chin. A round face follows a continuous curve with no corner to find. A square face has a distinct angle at the back of the jaw where the direction changes.
The thresholds used here put a round face near 1.06 for length against cheekbone width, near 0.90 for forehead against cheekbone, and near 0.88 for jaw against cheekbone. The jaw figure sits close to the square figure, which is why the curve test matters more than the number here.