This is a tool that you can't do a clutch/tranny install without it.
A clutch alignment tool is key to a smooth transmission install. I say you can't do it without, but I'm sure someone somewhere has. This tool makes it a lot easier.
It basically mimics the input shaft of the transmission. it has the same pilot diameter as the bearing or bushing in the back of the crankshaft to find true center. Then it has the same diameter and spline of the clutch to hold it in the middle while you tighten the pressure plate bolts. With the bolts tight, you shouldn't be able to move the clutch at all and the alignment tool should slide in and out easily.
These tools are usually made of plastic and come with most clutch kits. If not you can find them at your local auto parts store for like $4.