Hi again,
The resistor at the transistor emitter senses the current level. It converts the current into a voltage. That voltage then appears at the op amp inverting terminal. The op amp puts out a voltage that drives the transistor such that when the voltage at the inverting terminal equals the voltage at the non inverting terminal the op amp stops changing its output voltage. So the op amp tries to drive the transistor in a manner that will make the inverting terminal voltage equal to the non inverting terminal voltage.
When it accomplishes this, the current through the emitter resistor is approximately equal to the current through the collector and so that sets the collector current which is the load current.
The emitter resistor is typically small like 0.1 ohms but it could be higher like 1 ohm. You could also include a collector resistor to take up some of the power because the transistor ends up dissipating all the power. And of course the transistor needs a heat sink.
Also, since the output current level may be high you might need an additional driver transistor to drive the bigger transistor. That driver transistor can be smaller than the larger one. The connection would be base to op amp output, collector to big transistor collector, and emitter to big transistor base. When the smaller transistor conducts it pumps current into the base of the big transistor and that provides for more drive. The op amp output is limited to maybe 20ma max, and depending on the gain of the output transistor that may not be enough to drive the load so the smaller transistor helps provide more current into the base of the larger transistor.