That's heredoc syntax. You start a heredoc string by putting <<<
plus a token of your choice, and terminate it by putting only the token (and nothing else!) on a new line. As a convenience, there is one exception: you are allowed to add a single semicolon after the end delimiter.
Example:
echo <<<HEREDOC
This is a heredoc string.
Newlines and everything else is preserved.
HEREDOC;
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