A Here document is a set of lines in a script which is to be sent to a command as it's input.
It consists of
<<deliminator followed by multiple lines and then a line with just
demiminator to show the end of the document.
For example, we have a 4 line document that will be sent to the command as it's standard input:
1command <<TERMINATOR 2line 1 3line 2 4line $someVar 5line 4 6TERMINATOR
See also: Here strings