What does "it used the passive voice" (6th paragraph) mean?

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The indictment of longtime Donald Trump associate Roger Stone offered the clearest evidence to date of the Trump campaign’s alleged attempts to cooperate with WikiLeaks during the 2016 election.

Special counsel Robert Mueller embedded the tantalizing morsel near the start of the 24-page indictment. He recounts how the Trump campaign swung into action after WikiLeaks — the activist organization suspected of cozying up to Kremlin-backed hackers — started releasing stolen Democratic emails in late July 2016, just days before Hillary Clinton accept her party’s nomination.

In one paragraph, Mueller alleges that an unnamed individual gave instructions to a senior unnamed Trump campaign official to get in touch with Stone “about any additional releases and what other damaging information” WikiLeaks was holding about Clinton’s campaign.
After getting those orders, Stone allegedly told the Trump campaign “about potential future releases of damaging material” that WikiLeaks was holding.

Democratic lawmakers and legal experts tracking the Russia probe singled out those details — which suggest the Trump campaign willingly engaged with a foreign entity seeking to meddle in the presidential election — as the most alluring revelation yet in the Mueller investigation.

The revelation, if true, also suggests that Mueller is potentially sitting on more evidence that could firm up a case of collusion against at least some individuals in Trump’s orbit, or even the president himself.

“This indictment is significant because it alleges coordination between the Trump campaign and WikiLeaks,” said Barbara McQuade, a former federal prosecutor who called the language in the paragraph “particularly alarming” because it used the passive voice when describing the campaign officials.

“This language is very different from other language we have seen Mueller use,” she said in an interview . “He usually is careful to use some identifying language so that the person can be referenced easily. One reasonable inference is that the person who directed the senior campaign official is someone who cannot be indicted: the president himself.”

The White House so far has distanced itself from Stone, who was formally charged by a Washington D.C. grand jury on Thursday with lying to Congress and obstructing lawmakers’ investigation into Russian interference in the 2016 election.