Look at the left-hand side of the screen to find the "Labels" button with a little plus sign next to it.

Source: https://mashable.com/article/how-to-organize-gmail-inbox

The ability places a plus sign – with the sun hand, and a minus sign – with the moon hand.

Source: https://gamerant.com/hunter-x-hunter-hisoka-vs-chrollo-explained/

To use note pens on Mac, open a note in the edit mode and tap on the blue plus sign icon (+) in the note editor.

Source: https://www.devicemag.com/taking-notes-on-macbook-air/

You’ll need to wait a short while for a blue button with a plus sign to appear before you can post a comment.

Source: https://southafricatoday.net/business/buildrsa-national-conference-aims-to-find-tangible-solutions-to-tackling-south-africas-challenges/