What does it mean double arrows appearing when I hover over an instrument with the mouse?

Those double arrows mean you have to click and drag the knob to make it move.

This is typically used in VORs, altimeters, airspeed indicator, directional gyros and artificial horizons.

Most of these manipulators have been replaced by the new "SuperManipulator", which combines Click, Click-hold, Click-drag and Scroll wheel manipulation in one intuitive manipulator system.