To put it in aircraft terms, the systems on board need a certain voltage to work.
This is merely a potential difference between the power input line and power return line of (normally) +28Vdc, or 115Vac.
This difference can even be +14V on the input, and -14V on the return: the difference is still 28V and the + and - are purely with respect to a nominated reference, such as the batteries.
Imagine you have two 14V batteries and need to make 28V: You'd connect them in series (i.e. +ve on battery 1 wired to -ve on battery 2) such that a multimeter with the black lead on the negative terminal of one battery and the red lead on the positive of the other would show a 28V difference in potential between the two points. You ALWAYS need a reference point. In this instance, the black lead is the reference point, and the red shows the difference to that point.
Move the red lead to the middle point (i.e. the +ve of battery 1 or the -ve of battery 2) you'd see +14V with respect to the black lead (your reference point).
Put the black lead in the mid point and your reference point changes. Now if you put the red lead on the +ve of the second battery you'd see +14V with respect to the black lead . Move the red lead to the -ve of the first battery and you'd see -14V because the reference is higher than the point you're measuring.
You could connect 10 batteries in series, and measure the voltage difference between any of the points, such as -14V to +140V, depending on how many batteries lay between your black and red leads, and which way round you had them. If you do try this, beware; 10 batteries can flow a LOT of current!!
When the aircraft is flying, the airframe could be at -1000Vdc when compared to planet earth, but as long as the on-board system has a difference of 28V (in the right sense) across the power lines it will work quite happily.
Until aircraft came along, ground / chassis / earth were pretty much the same thing, as everything was attached to planet earth. Once we could escape gravity, voltages were no longer 'tied' to the planet and it's global earth reference and were free to wander about without any real effect to the onboard systems.
Does this help or have I made the waters even muddier?