Usually it means the professor is taking a break from teaching so they can focus on their research. There's no sabbatical police force though, so if the professor wants to hike the AT or backpack across Europe or something that's their prerogative.
Reif's e-mail [1] indicates he was forced into administrative leave, and that the leave was not Lloyd's decision.
> The actions of a senior faculty member have raised new concerns. In keeping with MIT practice on faculty discipline, I have asked his department head to consider any appropriate action. In the meantime, in consultation with the provost, dean and department head, I have placed him on leave.
Also I think administrative leave is different from other types of leave, which are all also different from sabbatical.
This is from 2020, the sabbatical GP and above are talking about was in 2017, as mentioned here:
> Eventually, in July 2016, Professor Lloyd affirmatively contacted Epstein by email to ask for funding to support his upcoming sabbatical. On June 1, 2017, Epstein emailed his accountant and Professor Lloyd: “send 125 k to mit for seth lloyd from gratitude.”
