How this works in practice:
A respondent navigates to the questionnaire, and we generate a session cookie for them. This identifies them as one respondent, and therefore helps us identify all the responses they leave as theirs. We generate a respondent ID for them and include all their responses in the results data.
If your questionnaire has the Maintain respondent session -setting switched on (it is by default), the respondent can leave the questionnaire, go eat some real cookies, and then come back to fill the rest of the questionnaire. Due to the data stored in the cookie, Maptionnaire is able to recognize that this is the same respondent who already answered some questions. They will therefore be able to continue responding to the questionnaire without having to retype their answers or without the same respondent being recorded as two different ones in the results data.