There is a built in variable called frameCount. This is the number of how many frames have been drawn with the draw() loop.
Here is an example that uses the frameCount to change the size of a circle. The circle gets bigger with each frame of the animation.
ellipse(width/2, height/2, frameCount);

p5 tries to draw 60 frames per second. So the circle will get big fast! After 5 seconds the circle will be about 300 pixels in size.