The mass
\[m\]
of a body is a measure of the 'amount of stuff' in a body.The weight
\[W\]
of a body is a measure of the force, specifically the force of gravity acting on a body.Despite being different concepts, mass and weight are related at each point in space by the equation
\[W=mg\]
where \[g\]
is the acceleration due to gravity at that point and is equal to about \[9.8 m/s^2\]
on the Earth's surface.The above equation ignores all forces except the force of gravity. It would not be true for a charged particle in an electric field because the force would be at least partially electric, so would not be due solely to gravity.