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When the 555 IC is used to produce an MONOSTABLE circuit - it will only pulse once. Monostable circuits can be used to turn lights/LEDs on or off just once. They are also used in many more school based circuits.
Look at the circuit drawn below. Pins 6 and 7 are connected and go to the positive (+9 volts). This is the easiest way of recognising that a 555 IC has been set up as monostable.

When the switch is pressed current flows into pin 2. Current then flows out of pin 3 switching the transistor. Current can now flow from +9 volts to -0 volts and the LED lights. In this monostable circuit when the switch is pressed the LED only lights once. The switch has to be pressed each time for the LED to light. This example shows the LED staying on for approximately 8 seconds. If the value of the capacitor is increased the length the LED stays on increases.