Do you want to show a simple tooltip when hovering over the legend item? This can be done in several ways:

Add a title HTML attribute:

When you want to show a simple tooltip for the legend you can create them using the HTML attribute. 

Set property useHTML to true and return inside labelFormatter HTML element with attribute title.

Demo: https://jsfiddle.net/BlackLabel/hd35vtkn/.

Render tooltip using SVG Renderer:

To create a more complex looking tooltip or to add styled options use SVGRenderer.

Demo: https://jsfiddle.net/BlackLabel/oc6npade/

Documentation:

https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/title

https://api.highcharts.com/class-reference/Highcharts.SVGRenderer