I have been reading: Dissecting Display, Chapter 1. Hello, Triangle!.
Then please read the next page; I explain exactly what it does there 😉
If I may quote myself:
We assigned the attribute index of the position attribute to
0in the vertex shader, so the call toglEnableVertexAttribArray(0)enables the attribute index for the position attribute. […] If the attribute is not enabled, it will not be used during rendering.