Both "to dip" and "to dunk" are verbs that describe the action of immersing something briefly in a liquid. However, there is a subtle difference between the two words. "To dip" generally refers to the action of briefly submerging something into a liquid and then quickly removing it. For example, you might dip a carrot stick into ranch dressing before taking a bite. The emphasis is on the briefness of the action. On the other hand, "to dunk" implies a more forceful action of submerging something into a liquid, often for a longer period of time. For example, you might dunk a cookie into a glass of milk, leaving it there for a few seconds until it softens. The emphasis is on the force of the action and the duration of the immersion. In general, "to dip" is used for more delicate items and a more gentle action, while "to dunk" is used for stronger items and a more forceful action. However, the distinction between the two words is often subtle and can depend on the context and the speaker's preference.

They both also have a wide variety of other meanings, but I assume you're asking in the context of putting something into a liquid or semi-liquid. You "dip" a chip, carrot stick or celery stick just below the surface of the dip or dressing to get enough to cling to the chip or stick that it adds flavor. Lots of people like to "dunk" a donut in their coffee. They put it down into the liquid and hold it there for a second or two so that the donut is soaked with coffee.

And of course, "dunking" a basketball means you get it over the rim before releasing it.

To add to Jonathan Kimball's excellent answer, to me "dunk" is a slightly informal word, and it suggests a vigorous action that creates a splash. For example, if you were using a Covid-19 test involving putting a test strip into a vial of fluid, you would carefully "dip" the strip, not "dunk" it. If you imagine people playing around a swimming pool, at the shallow end the little children "dip" their feet into the water. At the deep end, teenagers might throw someone into the pool, "dunking" them.