Turn both switches in the first circuit on the left (where the two transistors are in series) to turn on the lamp.
Turn either one of the switches in the second circuit on the right (where the two transistors are in parallel) to turn on the lamp.
The transistors are used like switches in these circuits. When two switches are in series, this creates an AND circuit. You must turn on both switches to send the signal. On the other hand, when two switches are in parallel, this creates an OR circuit. You can either turn one switch to send a signal. When both switches are turned on, the result is still the same as one switch is turned on.
