It's a suffix that can be added to words to change their function and meaning.
Here are some examples of how "ING" can be used:
* Verbs: Adding "-ing" can create present participles (e.g., "running," "swimming," "singing").
* Nouns: Adding "-ing" can create nouns that represent an action or process (e.g., "walking," "shopping," "learning").
* Adjectives: Adding "-ing" can create adjectives that describe something that is doing something (e.g., "interesting," "shocking," "frightening").
So, "ING" itself isn't a part of speech, but rather a morpheme that can contribute to different parts of speech when added to words.