From the letter:
In his resignation letter, which the White House emailed to reporters, Mr. Flynn said he had held numerous calls with foreign officials during the transition. “Unfortunately, because of the fast pace of events, I inadvertently briefed the vice president-elect and others with incomplete information regarding my phone calls with the Russian ambassador,” he wrote. “I have sincerely apologized to the president and the vice president, and they have accepted my apology.
It's from Yes Prime Minister - 'need to know'. I failed to tell you everything you needed to know because you didn't ask me the relevant questions. I didn't lie, nor deliberately mislead, just failed to tell you everything