I believe this is referring to the famous Sunday meeting with all of the centrist candidates and former president Obama. This happened right after Bernie won the South Carolina primary.

As a result of that meeting, buttigieg dropped out and endorsed biden (He got Secretary of Transportation)

Klobuchar also dropped out and endorsed Biden. (She got Chairman of the Rules Committee)

Warren, also at the meeting, stayed in the race to split the left-leaning vote.

This backroom meeting was the turning point for the primaries. Previous to this meeting, Bernie had a solid lead. Afterwards, it was all Biden.