SARS-CoV-2, the virus that causes COVID-19, is a RNA virus. It uses the RNA-dependent RNA polymerase to synthesis its genome. Unlike other polymerases, RDRP has no proofreading ability, meaning that if the polymerase made a mistake it cannot correct that. As a result, SARS-CoV-2 is constantly mutating. It is estimated that SARS-CoV-2 has 20.945 substitution mutation per year.
Mutations were used to trace the source of COVID-19 infection.
NextStrain has a nice visualization of all strains of SARS-CoV-2.
The SARS-CoV-2 VOC 202012/01 strain emerged in December 2020 from the United Kingdom and became highly prevalent in southeast England. The VOC strain has 14 substitution mutations and 3 deletions. The D614G mutation allows it to be spread faster than other strains. The N501Y mutation in the spike protein causes the VOC strain to bind tighter with the angiotensin-converting enzyme 2 receptor. The consequence of the tighter ACE2 receptor binding is not clear at the time of writing. N501Y and 69/70 deletion would likely change the shape of the spike protein.
While the mutations in the spike protein could affect the efficacy of monoclonal antibody treatment, it is less likely to affect immunity afford by vaccine or natural infection. Monoclonal antibodies are produced by same cells and targets the same part of the spike protein. The antibodies produced after vaccination are polyclonal, meaning that many different cells produce antibodies targeting different parts of the spike protein. Furthermore, in cell cultures, hamster serum with antibodies against D614 is known to neutralize G614 strain: Read here.
Although the faster spread of G614 strain is alarming, this mutation has been observed in Europe as early as mid-to-late January 2020. By August 2020, 44% of European samples in a study contain such mutation. Source. The fact that D614G is found in VOC strain is more an evidence of D614G's high fitness rather than the cause of the new strain.
So far we know that SARS-CoV-2 is constantly mutating, that G614 strain spreads faster than D614 strain, that G614 strain is no more severe than D614 strain, and that the vaccine most likely will work on the new strand. It is hard to say whether it is a good thing or bad thing based on what we know so far.