This is a very interesting question. In order to help you answer this, I would first of all suggest that you start by explaining what coronavirus is. You could do this by explaining that coronavirus, or COVID-19, is a certain strain of the COVID virus, which has suddenly started to spread across the whole world. The reason why this spread has been possible is because the COVID-19 strain is extremely contagious, and the human immune system does currently not have any immunity to this particular strain. As it is spread like any other cold virus, it has led to many people being hospitalized and dying.
I think it is fair to say that the main problem caused by COVID-19 is the number of people falling severely ill, with many dying after contracting the virus. This has put severe pressure on the health system, as hospitals and doctors struggled to keep up the care of their patients after such a huge rise in hospitalizations.
This is why the invention of the COVID-19 vaccine is without doubt the biggest contribution science has made in order to solve this problem. Through the vaccine, people are now less likely to contract the virus; even if they still catch it, they will have a much less severe reaction to it once they have been vaccinated.