That's the wastegate actuator line. One end connects to the compressor side of the turbo, and the other is connected to the wastegate actuator (seen in picture). The tee that you capped off is supposed to connect to a line that runs to the boost control solenoid (BCS) bolted to the front of the manifold.
Basically, the job of the BCS is to bleed off the pressure created from the turbine side, to control how much the wastegate opens up, this then controls how much boost the turbo produces. With it disconnected like that, you're only going to make what's known as "wastegate pressure", which is usually around 8psi.
Just peak around that general area and under the intake for a small rubber line that's not connected to anything, and re-secure it on that tee with a zip tie!