a burden: a load that is lifted and carried, whether goods on an animal's back or a weight laid on a person.
This is the plain, physical word for a load. It comes from the verb meaning to lift or carry, so a burden is literally the thing that gets picked up and hauled, the freight on a donkey, the gear the Levites shouldered, the sacks of grain brought through a city gate.
From that physical core the word stretches in two natural directions. Carried over to people, it names a weight that presses on someone who has to bear it: the strain of leading a complaining nation, the heaviness of guilt, a guest who has become hard to support. Carried over to politics, it names goods handed over to a stronger power, the load of silver or tribute that a weaker kingdom pays.
In every case the picture is the same. Something heavy has to be lifted and borne, and the word keeps the reader's eye on its weight and on whoever has to carry it.