It will set the value of guiTexture.enabled to false when your script is enabled.
If you don't have a Start function, you can create one by adding this to your script:
function Start ()
{
guiTexture.enabled = false;
}
The code inside the Start function gets called one time when your script gets enabled.
FYI:
The code you already have executes every frame because it’s in a function named Update. Every frame it checks whether or not the F1 key is pressed.
If F1 was pressed during the current frame, it executes the code in the if block, which toggles the value of the enabled property of the guiText attached to the GameObject with your script.