Whats the metal plate on some transitors with the big hole meant for?
For attaching a heatsink.

That big metal part is bonded directly to the semiconductor die inside the device, allowing it to efficiently conduct heat away from the die and to the outside of the package. The hole in it is for a screw, to mount it firmly to a heatsink. Since it's bonded to the die, the tab (as it's called) is also electrically connected to the device, usually but not always mirroring one of the functions of the pins (frequently ground). This can be important when several devices share the same heatsink.

Incidentally, this type is called a TO-220 package. There are other packages designed for heatsinks, as well; TO-247, for instance, is another particularly common one with a similar shape; it looks like this:



The TO-247 package is generally more effective at transferring heat (we engineers say it has a "lower thermal resistance") than the TO-220, but it's also slightly (as in fractions of a cent, but it still matters when you're buying millions!) more expensive to produce.

Some devices are also available in TO-220F packages, which are identical to TO-220 but with the tab insulated in plastic to avoid electrical contact to the heatsink; these look like this:



They are have much higher thermal resistance than the non-insulated kind, so you don't see them used very often.

There are other packages designed to screw onto heatsinks like this, and there are surface-mount packages with similar designs, but I think this answer is getting long enough already! Hope that helped.