How does a debounce switch work?

[S_Flaherty]

i recently had a lab where i needed to build a circuit with a debounced switch in it. now i have to explain how the debounced switch works but I'm having trouble articulating the different operations that occur.

the circuit consisted of two wires each with a resistor of the same type and both connected to a 5V source. the two wires are connected by the switch and each wire is connected to different parts of the same NAND gate. when the switch is in one position the output is 1 and the when it's in the other position the output is 0.

 

[Simon Bridge]

Guiding questions:
So what is the problem a debouncer is supposed to solve?
What causes that problem?

What happens with the gate when the problem occurs?
http://ikalogic.com/debouncing.php