Joomla language files: what does the %s mean?

Here is an english language file (en-GB.com_name.ini) to translate the JText(' ') class:

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SEND_MSG=Hello %s,\n\nThank you for registering at %s.\n\nYou may now Login to %s using the username and password you registered with.

SEND_MSG_ACTIVATE=Hello %s,\n\nThank you for registering at %s. Your account is created and must be activated before you can use it.\nTo activate the account click on the following link or copy-paste it in your browser:\n%s\n\nAfter activation you may login to %s using the following username and password:\n\nusername - %s\npassword - %s

SEND_MSG_ADMIN=Hello %s,\n\nA new User has registered at %s.\nThis e-mail contains their details:\n\nName - %s\ne-mail - %s\nUsername - %s\n\nPlease do not respond to this message as it is automatically generated and is for information purposes only.

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The %s constant gets replaced by variables, the question is:

What is the %s constant?
Where can I find it in the library?
or simply
What is this %s?

Thx,