Everything the practice writes about you — your intake, your consents, your exam, and the notes and photographs from every visit. It is in a vault that belongs to you, and this is the way in.
The design signs you in with your MeCentral account, and creates a free vault for you if you do not have one. That is not switched on for this practice yet, so the code above stands in for it. The code is weaker — anybody holding it can read your records — and it is temporary, and saying so is better than pretending otherwise.
Lost the code? The practice can send a new one in a moment — it does not cost you anything and nothing in your records changes.