An arc is part of the outline of a circle.
It can be used to add a smile.
noFill() stops the circle being filled in.
The centre of the arc is at the centre of the face.
It has a diameter of 300 (horizontally and vertically).
The arc extends from 0.25 on the right
clockwise (down and left) to PI - 0.25 on the left.
The angle unit is radians instead of degrees.
180° is PI (roughly 3) radians.
// smile
stroke(0);
strokeWeight(8);
noFill();
arc(width/2, height/2, 300, 300, 0.25, PI - 0.25);