A white Christmas is a Christmas celebration that is characterized by the presence of snow on the ground. An "official" white Christmas is defined by the Met Office as "one snowflake to be observed falling in the 24 hours of 25 December somewhere in the UK", but formerly the snow had to be observed at the Met Office building in London.
However, Climate change has brought higher average temperatures over land and sea and this generally reduces the chances of a white Christmas. additionally, with the average yearly temperature rising, one might think that the chances are reduced however what we are seeing is that the range of temperatures is increasing, aka when it's cold, it's colder than usual.