Much of what you'll hear today is based on James Comey's notes. He described in his advance testimony that he decided to begin taking contemporaneous notes of his meetings with Trump after becoming uncomfortable with the president's requests.
The FBI director began typing the notes in his car the moment he left Trump Tower and later the White House. He said he saw no need to take notes of the two occasions when he met Barack Obama.
Although they cannot be corroborated, contemporaneous notes taken by an FBI agent would hold up in court as credible evidence of conversations, so Comey's account is likely to be taken at face value today at the Senate.
"Contemporaneous" ("existing, occurring, or originating during the same time") was one of the most searched words on the Merriam Webster online dictionary on the day news reports about the meetings emerged.