VPg priming of the replication of RNA viruses
Nucleic acid synthesys can occur two ways, with a template or without. In most biological systems the template synthesis is the one that occurs since it is the only way to replicate genomic information. For template based nucleic acid synthesis in biological systems (being of DNA or RNA) the enzymes performing the reactions can use single stranded material (in the case of virus ssDNA or ssRNA virus) however they still need a small region of double stranded nucleic acid to bind and start polymerizing, otherwise synthesis could start anywhere in the template strand which wouldn't be particulartly useful, hence the primers, the polymerases bind to the double stranded region and start polymerizing always adding new nucleotides to the 3' end of the primer strand that are complementary to the nucleotide in the template strand. Usually in living cells the synthesis of new RNA molecules won't occur with an RNA template (there is usually no RNA dependent RNA polymerase, there are exceptions but they are usually result of previous viral infections), but only with a DNA template to procuce messenger/transfer/ribossomal RNAs. In the case of COVID and othe + strand RNA virus, the virus has to encode the RNA polymerase and also produce a primer that will create a double stranded region that will allow RNA synthesis to occur.