For example, from Bach's two part invention no. 11 (G minor):
That would be a "turn" - a common ornament in the baroque period.
You can find a (fairly basic) explanation here on Wikipedia,
and there are more detailed explanations in many books and articles on Baroque ornamentation.
You can also see Bach's own explanation here on the Dolmetsch website.
Here is Bach's own explanation, from the Dolmetsch website, showing symbols for different ornamentation figures that were used in this style period, with an example of how each might be played. The "turn" is number 4 on this chart.