From the Daily Telegraph (UK):

Why is the number ‘one’ so important as Covid-19 sweeps Britain?
The reproductive value describes the average number of people an infected individual can expect to pass the coronavirus onto

By Sarah Newey,
GLOBAL HEALTH SECURITY CORRESPONDENT
1 April 2020 • 9:41am

There are a lot of figures involved in understanding the coronavirus pandemic, be it around cases, deaths or tests. But there is one number above all others that experts obsess on – the reproductive value, or “R0”. …

But what does this actually mean and why does it matter?

The reproductive value describes the average number of people an infected individual can expect to pass the coronavirus onto. It is therefore a measure of how transmissible, or contagious, a disease is.

If a virus has an R0 of three, it means that every individual infected will pass the disease onto three other people if no containment measures are introduced. …

According to a pre-print study from the London School of Hygiene and Tropical Medicine published this week, the average number of people an individual comes into contact with each day has dropped by 73 per cent since the UK’s lockdown began.

“This would be sufficient to reduce R0 from a value from 2.6 before the lockdown to 0.62 during the lockdown, indicating that physical distancing interventions are effective,” the study, which tracked over 1,300 adults and has not yet been peer reviewed, concluded.