This is the clearest signal so far that Mueller is taking a serious look at Trump's business dealings.
Of course, this crosses the "red line" that Trump warned Mueller last July could not be crossed and that examining his business history would be a "violation" (details here).
For their part, Trump's lawyers have repeatedly warned Mueller and his people not to look at Trump's business history (details here and here).
Of course, Mueller is completely free to ignore such warnings and has the full force of the law and subpoena power behind him to do so. If anything, Trump and his lawyers' remarks unintentionally directed Mueller's attention toward Trump's business history.
What these subpoenas produce and when and if we'll ever see the results remains to be seen. If they're the basis for someone's indictment, more might become known when that unknown person is actually charged.
Until then, stay tuned.