Default setting

Cookies is enabled by default, the setting is "Allow from Current Websites Only or "Allow from Websites I Visit".

(Apple actually doesn't seem to document this)

How to change your cookie block settings

Block cookies

To set whether Safari blocks cookies in iOS 8 or later, tap Settings > Safari > Block Cookies and choose one of the following: Always Block, Allow from Current Websites Only, Allow from Websites I Visit, or Always Allow. In iOS 7 or earlier, choose Never, From third parties and advertisers, or Always. 

https://support.apple.com/en-us/HT201265