Somebody said they are different because of gravity.
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“Mass” is how much of something you have.
“Weight” is how much something presses down on a surface. Weight is defined as mass x gravitation.
On earth, weight and mass can be used interchangeably since gravitation is 1. On the moon, gravitation is 0.17.
‘1 gram of lead’ is an amount of lead sufficient to weigh 1 gram on Earth. If you take 1 gram of lead to the Moon, 1 gram of lead (referring to the amount) would press down on the surface of the moon with the equivalent force that 0.17 grams would press down on Earth while still being 1 gram of lead. It is literally 1 gram of lead which weighs 0.17 grams.
Another way to describe it is based on how much acceleration something would have given a force applied. So given a stationary piece of lead with 1 gram of mass and absent any gravity, if you apply an instantaneous force of 1 gram at 1 kmh, and assuming 0 friction losses and 100% energy transfer, the piece of lead should accelerate to 1 kmh. (Think like a cue ball hitting a billiard ball, the billiard ball shooting off and cue ball stopping dead.)
At its most basic, mass is the one measurement of the two you asked about which exists. Weight is an effect of the mass when acceleration is applied, specifically how much energy is required to accelerate it. (Btw, a gravitational field is indistinguishable from acceleration.)
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