A Phoneme Set is an asset used by LipSync Pro to determine what phonemes are available in your project. In this sense it's similar to the lists of Emotions and Gestures available in the Project Settings asset, but is a self-contained asset as a single phoneme usually forms one part of a coherent set, and breaking up a set doesn't make much sense.
The current Phoneme Set can be set in the Project Settings. Custom Phoneme Sets can be created from the Create Asset menu in the Project panel by going to LipSync Pro > Phoneme Set.
New phonemes can be added to the list by clicking the + button at the bottom, or removed by selecting one entry in the list and clicking the - button. Phonemes can also be re-ordered by dragging the handle on the left-hand side of an item up or down.
To the left of each Phoneme is its Index, determined by its position the list, and its Flag in parentheses. The Index is used to reference this phoneme in LipSyncData assets, and the Flag is used when filtering out Phonemes in the Clip Editor. These values cannot be set manually, but you should be aware of them to avoid breaking existing clips or poses in LipSync components.
On the right of the Phoneme's name is a Texture2D field for setting the guide image - this is the image that shows up in the lower-right-hand corner of the Scene View when editing a Phoneme pose. To the right of this is the Is Important checkbox. This is for marking Phonemes as visually important. While this doesn't affect animation directly, it is used by certain AutoSync Modules to protect important markers from being removed or modified.
Finally, at the bottom of the Inspector is the Scripting Name field. This is a simple string that represents this Phoneme Set for scripting purposes. It is used by AutoSync modules to determine which Phoneme Map to use when creating Phoneme Markers. In a custom Phoneme Set, this can be anything you like - just make sure it's unique and that you match it exactly if you also create a matching Phoneme Map.