1. var draw = function() {
2.    background(120, 240, 51); 
3.    fill(0, 0, 0); 
4.    ellipse(mouseX, mouseY, 50, 50); 
5. };

Line 1 is the basic draw function. Everything between this line and line 5 is in a function that animates the screen. It loops 60 times a second and draws the entire canvas every time.

1. var draw = function() {
2.    background(120, 240, 51);
3.    fill(0, 0, 0);
4.    ellipse(mouseX, mouseY, 50, 50);
5. };

Line 2 colours the background by mixing colours. The first number is how much red to mix, the second number is how much green to mix, and the third number is how much blue to mix. The range is from 0 to 255. 0 is the darkest, and 255 is the lightest.

1. var draw = function() {
2.    background(120, 240, 51);
3.    fill(0, 0, 0);
4.    ellipse(mouseX, mouseY, 50, 50);
5. };

Line 3 picks a colour to use when drawing things. In this case, (0, 0, 0) means no colour at all, which means black.

1. var draw = function() {
2.    background(120, 240, 51);
3.    fill(0, 0, 0);
4.    ellipse(mouseX, mouseY, 50, 50);
5. };

Line 4 is a command to draw a shape that takes 4 parameters. In this case, the shape is an ellipse, which is a circle. The first two parameters show where on the screen the shape is, first left to right and then top to bottom. The range for both is 0 to 400. The second two parameters give you the width and then the height of the object.

1. var draw = function() {
2.    background(120, 240, 51);
3.    fill(0, 0, 0);
4.    ellipse(mouseX, mouseY, 50, 50);
5. };

Line 5 closes off the function from line 1. Always remember to have closing brackets for every function.