"So without getting into the specifics, I can tell you that to the extent that investigation is a relatively important investigation and meaningful, the president would have been periodically briefed"

Robert Mueller is recommending that the president was kept notified of the progress of the examination. He is not offering any details, but he is indicating that the examination was significant enough to warrant the president being informed on it. This implies that the investigation was of a high priority and that the president was maintained to date on its progress. It also recommends that the president knew the examination and its implications. Mueller's statement indicates that the president was kept informed of the examination's development and understood its significance. This suggests that the president was involved in the examination and understood its implications.