I will post about this in more detail on another post. These things are easier to take in little bits.
A glucose molecule is unable to pass through a cell membrane via simple diffusion because it is too large. Instead, cells need to assist the glucose, which is why it is called “facilitated diffusion”.
A simple analogy is that insulin is like a “key” that unlocks the door for the glucose transporters.
At low levels of insulin, there are not many glucose transporters on the membrane.
Increased insulin causes the glucose transporters to fuse with the plasma membrane and “open” the door.
I will post more about this.