Viral vector-based vaccines differ from most conventional vaccines in that they don’t actually contain antigens, but rather use the body’s own cells to produce them. They do this by using a modified virus (the vector) to deliver genetic code for the antigen, in the case of COVID-19 the spike proteins found on the surface of the virus, into human cells. By infecting cells and instructing them to make large amounts of antigen, which then trigger an immune response, the vaccine mimics what happens during natural infection with certain pathogens - especially viruses. This has the advantage of triggering a strong cellular immune response by T cells as well the production of antibodies by B cells.
COVID-19 vector vaccines use a harmless non-replicating virus, which carries the gene of the COVID-19 spike proteins. After entering the human cell, the vector virus is unable to produce further copies of itself, it cannot cause COVID-19 disease and it does not enter the nucleus of the cell where our DNA (genetic material) is located, so it cannot change or influence our genes.