The intermix ratio was the ratio in which matter and antimatter were combined. A warp core is the most familiar device in which the intermix ratio was essential.
Following its expedited launch during the V'ger incident, the warp core of the refitted USS Enterprise required new fuel equations to balance the intermix. The essential simulation of the flow sensors was bypassed resulting in an antimatter imbalance. (Star Trek: The Motion Picture)
Several aspects of the intermix ratio are further explained in the Star Trek: The Next Generation Technical Manual (pages 61-62). According to the text, the 1:1 ratio applies when traveling at warp 8 or faster. When traveling at lower speeds the intermix ratio is different - a ratio of 25:1 is only used when the core is online in an idle state, prior to full startup. At warp 1 it is tuned to 10:1, gradually changing until it reaches 1:1 at warp 8.