That means add 7.4 gallons of 166F water to your grain. The grain will chill it some, and the temp will stabilize at 154F...this is your target mash temp. Leave it at that temp for an hour.
Then add 2.5 gallons of boiling water at the end to raise the temp of the whole thing to 167F...let it sit for 15 mins (this denatures the enzymes and stops conversion)
Saccharification Rest is a fancy term for mashing grain, where enzymes convert starch to sugar.