The measure tool, located in the layout workspace lets you measure the distance between two points on your model and change the scale of your model based on this measure. It’s a pro feature.
Click on the tool icon to launch it, there is no shortcut associated.
Move the green pointer on the model surface then click to place the first point, and click again to place the second point.
The option at the bottom of the screen lets you change the scale of the model in mm or inch based on the measure you previously did.
If you press alt before placing the second point the line between the two markers will be constrained to one of the axis based on the overall direction. It’s handy if you want to change the height of a model of measure based on a specific axis.