The flour is the whole body of the bread. It consists mainly of starch and some protein. Some of the starch is broken down into simple sugars, those are food for the yeast,(which mean the yeast will eat those sugars so it will keep the fermentation going)
The protein is the strength of the flour and it has a big influence on the overall processing quality. A normal baking flour strength is usually just over 12%. The majority of the flour proteins are made of the amino acids glutenin and gliadin, which means when the flour are mixing with the water it will form gluten.