The dark smooth plains of the moon are?

They are Seas. A long time ago when the Moon's core was still active, hot lava would rise from inside the Moon and fill these giant craters (they were called basins back before they were filled with lava). We of course call them seas today because, go figure, some kind of liquid filled them up (lava).

So they were basins before they were filled with lava and now they are called Seas because they were filled up.