An objective is the reason for doing a study, what the researchers want to accomplish. The objective could be to compare the efficacy of two interventions, to determine whether one is non-inferior, to determine incidence or prevalence of a condition or whatever else the researchers deem the purpose of the study to be.
Deeper dive
Objective is often confused with outcome and endpoint. See Objective vs. outcome vs. endpoint for an in-depth discussion of the differences.