Complex sentence: A spread footer foundation, which is typical in residential building, has a wider bottom portion than the load-bearing foundation walls it supports, which "spreads" the weight of the structure over a wider area for greater stability.
Split into simpler sentences: A spread footer foundation, which is typical in residential building, has a wider bottom portion than the load-bearing foundation walls it supports. This wider part "spreads" the weight of the structure over more area for greater stability.