A meeting transcript is the raw text Copilot works from in a Teams meeting. When transcription is on, each line carries the speaker's name and a timestamp, and all participants see a notice that the meeting is being transcribed. Afterwards you find it in the meeting chat under Recap, then Transcript. Copilot draws on the transcript and the meeting chat, and cites its sources, so you can click through to the moment something was said.
A meeting generally needs to be transcribed for full Copilot support, unless the organiser enables Copilot without transcription. In a recurring series, switching transcription on or off removes Copilot's earlier conversation history for that series. A transcript holds everything that was said, so for a confidential meeting decide deliberately whether it stays on.