I am reading Kobayashi's book "Differential geometry of complex vector bundles". More precisely, I am on section 2 of chapter 1, page 5.

Kobayashi is trying to prove that if $E$ is a vector bundle on some manifold $M$, with a flat connection $D$, then it admits a "flat structure" $\{U,s_U\}$ which consists on an open cover of $M$ and a local frame of $E$ such that the transition functions are locally constant.

In order to do this, he starts with some arbitrary local frame $s'$ and looks for functions $a:U \rightarrow GL(r,\mathbb{C})$ such that in the frame $s_U= s' a$ the connection $1$-form is $0$.

Therefore, if $\omega'$ is the connection $1$-form in the frame $s'$, what he is trying to do is solve the following equation for $a$

$$ \omega' a + da = 0. $$

He claims that solutions exists since the "integrability condition" for this equation is obtained by differentiating it

$$ 0=(d\omega') a -\omega' \wedge da = (d\omega')a + (\omega' \wedge \omega')a = \Omega' a, $$

which is true since we assumed that the connection is flat.

My question is what does he mean by the "integrability condition". Moreover, why is that the integrability condition for that equation? And, also, why can he use the fact $da=-\omega' a$ when computing it?

I think he might be using some form of the Frobenius theorem, since I know that it is what you use from a "global" point of view.

In any way, I want to know precisely in this context what he means by that "integrability condition", maybe it is just something basic or standard that I am missing.