Tool tip

A tooltip (also called emergent description) is a visual aid tool, which works by placing the cursor over a graphic element, showing an additional help to inform the user of the purpose of the element on which it is located.

"The tooltip is a variation of the help balloons and is a very used complement in programming and design, given that they provide additional information without the need for the user to request it actively."

from Wikipedia.

You should not misuse the "Tool Tip" effects. You do not have to make them very extensive or add very wide images. ToolTips only support a width below 300 px.