Whenever you browse the web in Safari, your iPhone or iPad logs the sites you visit, which allows you to easily go back and visit those sites again. Your iPhone and iPad also store cookies and other data, which is what allows you to revisit sites without having to log in each time.
For privacy reasons, or just as part of basic housekeeping, you may want to occasionally clear this data from your iDevice.
Go to Settings>Safari and then scroll down and tap Clear History. Verify that this is what you want to do and then repeat for Clear Cookies and Data.
If you don't want the sites you visit in the future to appear in your history or to be tracked by cookies, use private browsing in Safari.
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