P5 is a library of functions. A function is simply a block of code that does a task.
Think of it like a scratch block. A Scratch block has a name and usually takes in a value or parameter.
A function is the same idea. Each function has a name that we can use to call the function. Most functions take one or more parameters. Some functions take no parameters.
To draw a circle use can use the ellipse function, and give it 4 parameters.
For more on drawing circles, check out the Circles page.
We call a p5 project a sketch, as in sketchbook, because they are quick to start! For every sketch, two functions most defined for the sketch to run properly.
The p5 editor automatically includes these for you. You can change the values to change the canvas size and the background color.