What does the 'R' number of coronavirus mean?

The 'R' number of coronavirus is a figure that is being closely scrutinised as ministers decide when to end lockdown in the UK. Boris Johnson has said the country must keep the R rate below one in order to avoid the disaster of a second peak. But what does the R number mean and why is it important? Kate Proctor explains why the government is keeping a close eye on it and what it signifies.