How can I lower output impedance of an accelerometer?
One suggestion is to utilize a low input offset rail to rail op amp to
  act as a buffer to lower the output impedance, but I am not exactly
  sure how this works.


I would suggest...

...using a low input offset rail to rail op amp to act as a buffer to lower the output impedance. ;-)

At the most basic level an amplifier is more of something out than that something goes in, but depending on semantics, you can get the opposite if so desired.

The operational amplifier (OA) lets very little current flow in (ideally zero). To do this, it must appear to the source as a very large resistor between the input and the reference (typ. "ground") and isolate everything else down the line. 

On the output side, the amplifier provides an output voltage equal to the input voltage multiplied by its internal gain (feedback can modify the gain) independent of whatever else is connected to the output (ideal case, but practical OA's get fairly close). 

Therefore to the load, looking back at the output of the amplifier, the OA appears like a very small resistor (ideally 0) in series with a very powerful voltage source.

So there!

Consequently the input has high resistance (impedance in the complex case), the output has low resistance, and you have your impedance transformation function.

Pragmatically, the amplifier has simply separated the ADC from the accelerometer by showing the accelerometer its input side (high impedance so the accelerometer output is happy) and the ADC its output side (low impedance so the ADC input is happy).

Hopefully, that helps you intuit through the terminology. Cheers!