Trump's eldest son, Donald Trump Jr., hosted a meeting with a Russian delegation in 2016 after receiving an offer via intermediaries of damaging information about Hillary Clinton.
Although the Trump family has changed its story about the meeting, the president's position since has been that Trump Jr. did nothing wrong and that it is fair game in politics to meet with people who make these kinds of offers.
At the same time, Trump has said he did not authorize or know about the meeting beforehand. The special counsel's investigators likely want him to address that for the record in their questions.
Congressional investigators asked Trump Jr. about the contacts he was making in the early summer of 2016 as he set up the Trump Tower meeting in New York City. He made two phone calls to a blocked number. Trump Jr. told lawmakers he didn't remember whom he called. Other witnesses said that Trump Sr. used a blocked number.
If investigators were to establish that Trump Jr. had called his father at the time he was scheduling the meeting with the Russians, it might undercut the president's denials. And if the president declined to answer a question about that to Mueller under penalty of perjury, that too could be suggestive.