When a visitor arrives at a HubSpot form, HubSpot checks the visitor's browser for tracking cookies. If this cookie is associated to a contact, HubSpot pre-populates fields on the HubSpot form with known values by pulling the information from their record.
These are the likely scenarios where your form fields are pre-populated:
If you previously submitted a HubSpot form, and this form tracks cookies, the entered information will be pre-populated when you visit a HubSpot form from the same account again. The form recognizes your browser cookie and pulls the associated contact record's information into the form fields.
If someone used your browser to submit the form and updated it to their own values, their information will overwrite yours. Hence, when you visit the same form again, their information will appear instead.
If your form fields pre-populate with another person's information who has never submitted a form on your browser, it is likely due to a forwarded email. This is likely what occurred:
You forwarded an email to someone.
They click a link in that email to arrive at a HubSpot form. This link in the email is tied to your contact record, as the email was sent to you.
They submitted the form with their own information. Because the link in the email is associated to your contact record, this form submission now will update your contact record with this information.
When you visit a HubSpot form from the same HubSpot account, the cookie in your browser identifies your contact record and pulls up the information in your record. Because your record has been updated with the person's information, this information is now pulled up.
If you (or your contacts) are noticing another person's information pre-populating your form fields, clear your browser's cookies and re-submit your form to become associated with a new tracking cookie.
Alternatively, you can add a link for users to reset your form in your form options. Clicking this link will reset the form and disable cookie tracking in the form submission, which prevents any cookie overwrites.