In-vitro test is one the initial test perform on any drug to measure its safety as well as its positive/negative effects. in this test the healthy body cells are cultured in a petri dish where the conditions (e.g. temperature and medium) are kept similar to normal body conditions.
Afterwards the under-study-drug is added to this solution in order to confirm its safety for healthy cells.
In the next experiment, the target virus or bacteria are added to the healthy cells. Afterwards, the research drug is added to the solution containing body cells and virus. This way the effectiveness of the drug in preventing the virus growth can be measured.