What happens when your penis becomes loose?
If you are referring to "loose" as compared to your penis as a child, what it means is that you have reached puberty and you are in your adolescent stage of development. An adult male's penis is normally loose or flaccid. Since it is larger than your penis as a child, it hangs down. The skin on the penis is bigger than what you might think it ought to be. This is because shortly (usually within two years of reaching puberty) you will develop the capability of having erections.
An erection is when your penis swells and becomes longer and wider. The swelling makes the penis rigid, much like blowing up a balloon makes the balloon stiff. The stiffness in a penis does not come from air, but from blood swiftly entering the penis, but only slowly leaving it. Since the penis is larger when it is erect (just as a blown up balloon is bigger than an empty balloon), the skin on the penis must be loose so that it doesn't cause pain when the penis swells up during an erection and stretches the skin.