Examples of commercial use include
- Selling a song on bandcamp
- Putting a song on a streaming service like Spotify
- Enabling advertisements on an UTAU video and collecting ad revenue
- Making subscription-only songs with that UTAU posted on Patreon
- Selling a physical CD of songs with the UTAU
- Creating merchandise of the UTAU, like stickers, prints, or keychains
- Using the UTAU to promote a paid product or service
There are other possibilities besides this list, but this should begin to give you an idea. Essentially, it's using your UTAU to earn money. You might want to give complete freedom to users to do whatever they want, or you could restrict it by saying they need permission, or need to buy a license, or need to share a percentage of the money with you.