See the latest R number range for the UK here:
What is R?
The reproduction number (R) is the average number of secondary infections produced by 1 infected person.
An R number of 1 means that on average every person who is infected will infect 1 other person, meaning the total number of new infections is stable. If R is 2, on average, each infected person infects 2 more people. If R is 0.5 then on average for each 2 infected people, there will be only 1 new infection. If R is greater than 1 the epidemic is generally seen to be growing, if R is less than 1 the epidemic is shrinking.
R can change over time. For example, it falls when there is a reduction in the number of contacts between people, which reduces transmission.
Why is it important?
The Government has explained in its roadmap for re-opening the country that one of the 5 Tests is that the R number should be 'manageable'. The rhetoric around this has consistently been that the R number should be below 1.