Twinscroll is the turbo type on latter Subaru STIs. I've taken a look at a few.
In the most basic terms it could be considered two turbines housed in one turbo unit. A small one that will spool quickly from low revs but will lose the will to live at, for example, 4k revs, and a big one that spools slower but sustains boost from, say, 4k revs through to red line.
The effect is quicker spool and boost sooner and longer. The payoff is increased cost; like most things.