Interpreting rating label of DC power supply - supposed to be a constant voltage source or a constant current source?
12 V is the value of the regulated, stable output voltage.  1000 mA (or 1 A) is the maximum output current you can draw and still expect the output voltage to remain at 12 V.  Note that the max output power is 12 W.  This is consistent because (combining Joule's Law and Ohm's Law), power equals voltage times current.

Another note: most switching power supplies cannot maintain regulation all the way down to zero output current.  A typical minimum load is 10% of the rated max.  In your case, this means that for loads less than 100 mA, your supply's output voltage might be something other than 12 V.  You can test this by connecting resistors of 120 ohms or more to the output and measuring the output voltage.  Be sure to use resistors rated for sufficient power.  The power dissipated in a resistor can be calculated using Watt's Law:  P = E^2 / R.  For safety, use a resistor rated for at least twice the expected power dissipation.

A simple resistor divider will not work as a step-down regulator for an external device because that device expects the supply voltage to remain constant asn the current load varies.  The 8266 draws more or less current depending on what the circuit is doing, and this will cause its supply voltage to vary.  Not good.  Best to add a simple voltage regulator circuit to step the 12 V down to what is needed and hold that output voltage value constant.