It basically means they are just giving you the vaccine and as part of the study they will not be exposing you to Covid-19 on purpose.
If they do the trial in large enough numbers, many participants will get exposes in their normal lives anyway. I assume they will target hotspot areas.
That is a thing that people do sometimes:
https://en.wikipedia.org/wiki/Human_challenge_study
There has been a debate about also doing it with COVID-19:
https://en.wikipedia.org/wiki/COVID-19_vaccine#Proposed_chal...