So this is about to design anti COVID-19 drugs targeting 2 molecules : the main protease and virus receptor binding domains. What is that? Let us get a little bit more details.
1. The main protease.
Protease is an enzyme that cuts proteins. 'ase' is a common ending of enzyme names and more on this is here.
Covid-19 main protease is a peptide and an enzyme that human or other host cell produces based on COVID RNA genome.
So potential dugs against the main protease will obviously work after virus RNA is inserted into the cell and starts it's pathologic activity.
2. Receptor binding domains
Receptor is a peptide that receives and transmits signals. Many of receptors are located in the cell membaine and acessible on the cell membrane surface. These are getting signals and virus can also bind to these. More reading on this here.