Viruses mutate. Period!
Fortunately, the mutation rate of the coronavirus is lower than other types, such as flu. However, due to the high transmissibility and fast-spreading of SARS-COVID-2, new "variants of interest" that might be more dangerous are appearing around the world.
I can stress enough the fact that the virus CAN NOT reproduce by itself, it needs an animal cell, it needs to hijack the cell's mechanisms to be able to copy its genetic material and to produce the viral proteins, it even takes a part of the cell membrane with when "leaves" the cell.
So, the first variant of interest was reported back in September in Kent (UK), as a chronic case (long extended infection), and its transmissibility was higher than the "original" variant, approximately 63%. it took some time to process all the data of the genome sequences and analyze the results, but now we know that the new variant is more contagious. The main change is an amino acid (like a building block/brick) in the spike/membrane protein. This protein is responsible to bind to a protein located on the surface of the animal cell, which starts the infection process. Apparently, this change (mutation) of just one part of the spike protein gives the virus an advantage that translates to a higher infection rate.
Other variants, for example, the one reported in Brazil or the one reported in Sudafrica have additional changes in the spike protein. One of these changes has shown, in the lab, to lower the immune response competence, due to a lack of recognition of the viral proteins.
The good news is that some vaccines are already being tested and are still effective against some of the variants, and in case that a new variant appears, the vaccine's mechanisms and technology can be adapted in a relatively short period of time.
The best that you can do and avoid being used by SARS-COVID-2 as a "reproduction" hub: Stay home, limit interactions with people from other households, keep 2 meters apart (physical distance), use a mask, wash your hands, and if you get offered a vaccine, don't miss the shot!
https://www.youtube.com/watch?v=i5RZMhxfVhg
References:
https://www.thelancet.com/journals/lanres/article/PIIS2213-2600(21)00005-9/fulltext
https://www.bmj.com/content/372/bmj.n579
