Checksum Tokens give your constituents access to view and modify their information without logging in. This reduces your constituents' time entering data, increases their response rate and improves the quality of your data.
Add them to links sent with CiviMail. You can add them to contribution forms, profiles, petitions, webforms and event registration forms that are prefilled with information that is already in your database’s contact record. This special link lasts for 7 days from the time it was sent out.
If you have a webform, then all you need to do is add this to your CiviMail*:
example.org is the name of your org
update-your-details is the name of your webform.
*For checksum instructions for Contribution Pages, Profiles, Registration Forms, and Petition Pages, check out this wiki page.