Consider the circuit shown. In a circuit with a LED and resistor R2, you are adding the NPN transistor BC 547 B. The emitter is connected to the negative terminal, the collector is connected with the LED.
The transistor has a third terminal, the base. The base should not be left unconnected, so you connect it with resistor R1 to the negative terminal of the battery. This puts 0 V to the base.
When you now connect the battery to the circuit, the LED remains dark. There is no current flowing through the transistor from the collector to the emitter. The transistor is "locked".
In the next experiment you "open" the transistor.