please explain this formular

Define B=B(y+y)B(y) Due to Taylor expansion, we have B(y+y)=B(y)+B(y)y+2!1B(y)y2+B(y)+B(y)y if we ignore all higher-order terms of y. We note that B(y)=DB thus we have B=B(y+y)B(y)DBy