Default responses can be designated in the question editor. The respondent will then see these values as pre-selected or pre-entered when they take your survey. The default response will be pre-populated into the question the first time the user sees it until they either submit new values for that question or the question is marked as valid. Default values will never appear as part of the response unless the respondent has seen the default response and submitted the page that contains the default value.
Assume you have a Single Text Field question that has a default value of 'Enter your response'. This question is not required and the respondent may skip over the page that contains this question (because forward jumps are allowed). If that happens, the response will be submitted without an answer to that question. The default text will not be part of their response, because they never submitted it as part of their response and you allowed them to skip that question by failing to mark it required.
However, let's assume the same situation as above exists, but you mark that question required. If they skip over the page and then submit the survey, the respondent will be returned to that Text Field question and prompted for a response, as they did not supply one. The default text will be pre-populated into the question, and they can either submit that or change it to something else.
You should also note that if you populate a response that results in a valid response to the question, the default values for that question will never appear.