If we want to know how much something weighs we need to determine its weight using a special measuring device called scale.
Historically the first scale was only a simple beam with two strings on each side and a plate attached to each string.
To determine the weight on one side, parts with a known and defined weight where put on the other side as long until the beam comes back into a horizontal position.
Nowadays we use mechanical or electronic scales.
The later converts the weight into a deformation, that can be tracked by a strain gauge and hence leads to a voltage difference.