Rick's weapon is a metal plançon à picot he calls Thrasher. This weapon, which shall hence be referred to as a plancon, looks like a basic club, though the end is covered in spikes, and the center has a spike sticking out of the top. A example can be seen Here. The inside of the placon has the workings of a gun, and bullets can be shot out of its pointed tip. Dust can also be infused into the weapon or into the bullets to give it a bit of an extra 'kick'.
This weapon can transform into a shield, and does so by breaking apart at the handle to form 'leaves'. These leaves are pushed straight upwards and with a twist lock together, creating a spiked shield. The shield can be charged with Dust, but the gun is useless in this form.
The weapon can be collapsed back into it's plancon form, though it can be pulled down around the arm holding the shield as well. When this is done, the arm has to be fully extended and won't bend easily, and the leaves of the shield fall back as if they were going to form the plancon again, trapping his arm inside it. It helps prevent a bit of damage to the arm, and can be infused with Dust, but the gun is still non-functional.
Essentially, it allows him to touch any clastic sedimentary rock and pull it onto his body, covering his epidermis in a thin layer of stone. This helps to protect him from attacks, though it is a bit hard to move in. There are a few notes to be made about this Semblance; the amount of clastic stone available limits whether he can use his Semblance or not. He can only cover his skin with whatever's available and only to the degree of how much solid is there. He has to touch the solid and take time to pull it over his skin, meaning that it takes time to get a covering up. The Second Skin is only as strong as whatever type of clastic stone he covered himself in, and he can still be injured through it.