。
Noodles are usually made from wheat flour.
Literal
Noodles [topic-は] usually wheat-flour [from-から] are-made.
A standard 'X is made from Y' construction in passive form. から marks the raw material or ingredient — as opposed to で, which can also mark material but with a slightly different nuance of 'made of Y.' Japanese loosely uses から for transformation cases ('Y becomes X') and で for composition ('made of Y'), though the two overlap.