Depth of Field (DOF) calculator
You can explain all day about the relationship between f/stop, aperture, and focus depth (depth of field) but nothing helps more than an interactive tool. You can either 1) go out on the deck and shoot 100 shots with different settings, remember the settings and remember what they do or 2) Play with this interactive DOF calculator to see just what happens when you change one or more settings on the virtual camera.
Best part is you can use the DOF calculator at work or at night when you really can't be taking photos and chimping them in the LCD. Point your browser at: https://dofsimulator.net/en/ From their site: " This application interactively calculates the camera depth of field and background blur and visually simulates it on a photo together with different types of lens blur (bokeh) for any lens, camera and distance combination."
Works best in late models of Firefox and Chrome.
tl;dr: Small lens opening (big f/stop) means deep long focus. Big lens opening (little f/stop) means very shallow focus and blurry background.