With both PCR and LFT, there is a period of time after you are exposed to the virus when the test will show negative but you are actually infected. The virus has not yet had the chance to multiply enough to register.
That period is shorter for PCR, but still not zero. So as pp noted, a negative test should not be taken to mean you are ‘safe.’ But then everything you do has some risk.
There is no absolute guarantee - all the measures we have are about reducing the risk of transmission. We cannot totally eliminate it. We need to get our heads round that and then take reasonable steps depending on the level of infection in the community.
I mean, if one person in 30 is infected, there is a significant chance you will meet (or be) one of them. If it’s 1 in 10,000, not so much.
Personally I am following whatever rules remain in place for as long as they do. But when I get to see my mum in a few weeks time, what with her having had two jabs and me having had Covid plus one jab, and probably my second as well, I dare say there will be hugging.