electrical conductivity
Some materials let electricity flow easily; others fight it. Electrical conductivity is the number that captures how willingly a material passes current — high for copper, almost nil for rubber. It's the opposite of resistance: a great conductor offers little resistance, a poor one a lot.
In the free-electron picture, applying a voltage gives every wandering electron a gentle, steady nudge in one direction, so the whole electron sea drifts and current flows. But the drift can't speed up forever — every so often an electron collides with something and loses its forward progress. Conductivity ends up set by how many electrons are free to carry charge and how long, on average, they travel between collisions.
It matters because conductivity decides what we make wires from and underlies all electronics. The honest caveat: the free-electron model treats the cause of collisions as a vague "something," and gets the temperature dependence only roughly right. Understanding what electrons really scatter off — vibrating atoms and impurities — needs ideas beyond this simple model.
Copper conducts electricity by something like a billion billion (10^18 or more) times better than glass. That vast gap is why power lines are copper and window panes are not — yet the free-electron model treats both kinds of material with the same simple equations, just with very different numbers plugged in.
Copper outconducts glass by an astronomical factor — hence copper power lines.
Conductivity (how easily current flows) and resistivity (how much a material resists) are simply each other's reciprocal — the same fact told two ways. Either is fine; just don't confuse the two when reading a number.