What does ".1M/250" mean as a capacitor rating?
That's a 0.1uF +/-20% mylar film capacitor rated for 250VDC. That is a typical capacitance, type of capacitor, and voltage rating for the capacitor used in a dimmer. The voltage rating is a DC rating. 



The capacitor actually only sees the diac breakdown voltage (typically 30-40 volts) plus a volt or two for the triac gate. 

The "M" does not stand for microfarads- rather it is a tolerance code. 


J = +/-5% 
K = +/-10%
M = +/-20%




In light dimmers, sometimes the diac is integrated into the triac as one unit.