Not a teacher.
The idea is that your opinions ("where you stand") depend on your station in life ("where you sit").
The king sits on a nice throne; the peasant may not have a chair to sit on at all.
The boss gets a comfy chair; the workers sit on hard benches.
If you live in a fancy house isolated from the city, you may not share the concerns of someone who lives in a crowded area.
I believe the saying you are referring to is "What you stand for ..."
If you "stand for" something, you believe in it, or strive for it.
A concrete example of this can be seen in most national parliaments in democracies. If you sit on one side (all members of the same party sit on the same side of the chamber), then you "stand for" big government, big taxes, social welfare, socialism.... If you sit on the other side, you stand for small government, private enterprise, capitalism, etc.
If you sit on the fence, you stand for nothing.
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