According to the Met Office's "What does the forecast mean?", it means "that [there is a 95% chance that] precipitation will fall at some point during that period [14:00 to 15:00]" and that we expect that precipitation to be heavy rain.
In other words, no information is provided for the expected duration that the rain will last, only that it is 95% likely that there will be some precipitation. It could be a 2 minute shower.
I personally find this "chance of precipitation" statistic useless. An hour is far too long and I'm not going to avoid going outside because there is a chance a shower might occur in that time slot. At most I'll change my behaviour by bringing a coat but in winter I do that anyway because it is cold. The only statistics I'm really interested in is expected rainfall and the Met Office doesn't provide that data.