Select the two objects you want to measure between and hit ‘link’.
The new ‘graphic’ is now an editable parameter on the right hand panel. This is also where you can delete it.
To measure between vertices, in Edit Mode select the 2 vertices and then hit ‘segment’
There are other parametrers you can measure too, hover your mouse over each for a tool-tip.
NOTE: by default the tool uses the origin points of the meshes you select. If however a vertex is already selected in an object, even in Object Mode the vertex will be used as the measure point. This way you can measure between an origin point of one object to the vertex of another.
hope that helps. There a lots of options ( explore ), but these are the basics.
If this info is just for your benefit in modelling etc, and you don’t need it to render out, then create two small boxes, or two empties and create a measureIt between them. Then you can simply place the boxes/empties where you need and instantly read off the displayed value - it’s all updated in real time.