Whenever the inputs are set to the low state which is ground. The input transistors are in saturation and the current flows through their emitter to the ground. When the inputs are set to high which is 5V, the input transistors do not let the current flow to ground.
When the inputs are set to high, the current flows through the output transistor and the voltage drop across the right side 3k resistor makes the output go low.
When either input is set low, the current will not flow through the right side of the circuit, which makes the output go high at 5V.
Because of the behavior of the circuit, this is a NAND gate. The output is only low when both inputs are set high.
