Lead is a raw resource used primarily in manufacturing Munitions.
Lead Mines produce 0.25 Lead per turn each, accumulating to 3 Lead per day. Lead Mines are reasonably cheap to build and operate, and never run out of Lead to mine. Each city can have up to 10 Lead Mines.
There are no National Projects that increase Lead production. To increase Lead Production, a nation must increase the number of Lead Mines it has.