The LA Times reports that the investigation is exploring her role in a meeting with Russian operatives, specifically lawyer Natalia Veselnitskaya, at a secret meeting with the Donald Trump presidential campaign in June of 2016. While Ivanka did not attend that meeting, she exchanged "pleasantries" with the Russians as they exited Trump Tower. She is not accused of any wrongdoing by the special counsel at this time.
Veselnitskaya is being asked to return for question by the Mueller investigation. The Mueller team reportedly want the details of every contact the Trump campaign, insiders, and family circle had with the Russian envoy on that day, and that includes Ivanka herself.
The secret meeting with Veselnitskaya and other Russians took place at Trump Tower in June of 2016 with Jared Kushner, Donald Trump, Jr., and other senior officials. This meeting was not reported to the public by the campaign and was only revealed after an investigation by the New York Times. The news that Ivanka was even in proximity to the meeting is new — the president's daughter has not previously been mentioned in connection with the Mueller investigation in any way.
The Trump administration said that meeting was only convened to discuss issues related to child adoption between Russia and the United States. The Trump campaign was offered incendiary information about Hillary Clinton by the Russians as a pretext for the meeting.
The timeline of the investigation is complicated, so here's a great primer to get you started if you're newer to this story. The special counsel was appointed in May 2017 by Deputy Attorney General Rod Rosenstein to coordinate the investigation into the 2016 election: namely, to answer two questions: did the Russians hack into the election, and if so, did they have help from Americans?