Usually appears when you position the mouse pointer on a toolbar button?

AnswerIf you mean when a text box appears with text in it then it would be the Alternate text.

Also when moving the mouse pointer over images too.

Alternative text serves several functions:

• It is read by screen readers in place of images allowing the content and function of the image to be accessible to those with visual or certain cognitive disabilities.
• It is displayed in place of the image in user agents (browsers) that don't support the display of images or when the user has chosen not to view images.
• It provides a semantic meaning and description to images which can be read by search engines or be used to later determine the content of the image from page context alone.