To pan, click and drag or use the arrow keys. On mobile, touch and drag.
To zoom, use the mouse wheel or touchpad, or '=' and '-' (you can also double-click to zoom in). On mobile, pinch open / closed.
Hue index and hue multiplier control the display color. If a point is in the Mandelbrot set, its color is black. Otherwise, if it diverges after n iterations, its hue, defined as an angular position in a color wheel, is computed as (hue_base - n * hue_multiplier) mod 360. Intuitively, hue index just changes colors, whereas hue multiplier controls the contrast between closely adjacent colors.
Max iter limits the number of iterations used to determine whether a point is in the Mandelbrot set (whether the Mandelbrot sequence at that point is bounded). Increase the value to see more detail as you zoom in. Also see what happens as the value approaches zero!
All plots are bookmarkable: the URL encodes everything necessary to reproduce the image.
For further reading, start with the Mandelbrot set Wikipedia entry.