The title attribute of an HTML element is used to convey additional information about the element. It is commonly used with images and is often shown as a tool tip when the mouse hovers over the object.
In HTML5 the title attribute can be used with any HTML element. However it is not well supported by many devices and therefore should not be used as a replacement for the image alt attribute.
Notes on usage from W3.org HTML 5.1 document:
Relying on the title attribute is currently discouraged as many user agents do not expose the attribute in an accessible manner as required by this specification (e.g. requiring a pointing device such as a mouse to cause a tooltip to appear, which excludes keyboard-only users and touch-only users, such as anyone with a modern phone or tablet).