The time signal on BBC Radio, consisting of five short pips and one longer one on the hour, the exact hour beginning at the start of the latter. Until 1990 they were officially known as the Greenwich Time Signal.
When a leap second occurs (exactly one second before midnight), it is indicated by a seventh pip. In this case the first pip occurs at 23:59:55 (as usual) and there is a sixth short pip at 23:59:60 (the leap second) followed by the long pip at 00:00:00. The leap second is also the explanation for the final pip being longer than the others. This is so that it is always clear which pip is on the hour, especially where there is an extra pip that some people might not be expecting. Before leap seconds were conceived the final pip was the same length as the others.
It is frowned upon at the BBC to talk, play music or otherwise make noise while the pips sound, and doing so is commonly known as crashing the pips.