AKA current mirror or current follower.
Note how initially the LED fails to emit due to the low impedance path available from its source through the circuit on the left.
Toggle the switch to enable the opamp and transistor. These function as a replacement for the 33K resistor while simultaneously preventing current flow from the right circuit to the left circuit.
This circuit allows a high impedance circuit (right side) to follow (mirror) changes in current from a low impedance circuit (left side). Current buffers are the counterpart to voltage buffers. The difference is which side has low/high impedance.
