As cityslicker mentioned, an open label trial is one in which clinicians and patients know who is getting what.

These are used to compare treatments or to identify appropriate patient groups.

If I understood correctly - in the case of covid, the nature of the emergency, the unique requirements of keeping one of the vaccines super cold (making it harder  albeit not entirely impossible to blind), the short time to death, etc. all led to discussions of various trial designs to ensure safety while allowing for warp speed.

An open label study also seems to make it easier and faster to switch treatments or discontinue immediately in the event of adverse effects or non effectiveness, which is desirable given the short term to death faced by severely ill covid patients.

Also, a disadvantage of open label studies is that they can lead to reporting biases. However, the rapid progress to mortality and to cytokine storm that result from severe covid seem less susceptible to certain kinds of reporting bias than the symptoms of some conditions.

I believe Don had mentioned that most/many covid candidates were starting with open label studies, but would have to check the transcripts to be sure.

In any case, this article may shed some light. https://www.sciencemag.org/news/2020/12/makers-successful-covid-19-vaccine-wrestle-options-many-thousands-who-received-placebos