Make a object instantaite 25 units in front

How do I make a object instantiate 25 units in front of my player. Can you explain how to add 25 units with vector3.forward.

Just multiply 25 to your forward vector. In this case I guess you want to use the transform.forward rather than Vector3.forward (unless you want it to appear 25 units in front of it, in world coordinates).

var clone = (GameObject)Instantiate(prefab);
clone.transform.position = transform.position + transform.forward * 25;

along a certain axis? (the z-axis) or along the forward direction of the player?
with z, you simply set the z-part of your instantiate position to 25, with the .forward of the player you take “the players .forward * 25” as the instantiate position