Trump's son-in-law, Jared Kushner, is already being questioned by the FBI, per media reports earlier in the week, and had a number of secret meetings with Russian officials that he failed to disclose during his security screening process.
Now Trump has repeatedly insisted there was no secret collusion between his campaign and the Russians. Then why was his son-in-law secretly talking to the Russians, in meetings he failed to disclose to federal investigators, and talking about a clandestine communication system meant to escape detection?
You don't have secret talks unless you have secrets to hide.
The Logan Act is a longstanding federal law that provides criminal penalties and prison time for people convicted of unauthorized communications between private citizens and foreign governments in adversarial positions with the United States. This law was passed to prevent exactly the sort of behavior in which Trump's people were engaging.
While Trump is now President and such foreign contacts are allowed, before he was inaugurated, he was still a private citizen bound by the Logan Act, as were members of his family and all of his employees.
As another article in The Washington Post noted (link here), this latest development about "Kushner trying to secretly talk to the Russians is the biggest billow of smoke yet" in the conflagration consuming the Trump administration.