When you want to use a variable from another script, you have to specify that script first. However, the compiler cannot understand the script that you’re trying to “inspect” on its own.
I’m supposing that the first script that you’ve posted is called “script1”, the second one is called “script2”, and script1 is assigned to an object called “my_object”. So, try this (in script2):
function Start(){
var my_prefab=GameObject.Find("my_object").GetComponent(script1).prefabs[0];
var instance=Instantiate(my_prefab, transform.position, transform.rotation);
}