A dot is a round speck or spot. A first grade teacher might remind his students to put dots on their is and js.
A period, a little circle or point, is a dot. Another kind is a polka dot, which might decorate your favorite pair of shorts or the curtains in your room. In Morse code, a dot is a short signal, as opposed to the longer "dash." When you recite your website's URL, you might end with "dot com." As a verb, to dot is to mark something with a dot: "You forgot to dot your is!"