A here string can be considered as a stripped-down form of a here document.
It consists of nothing more than COMMAND <<<$WORD, where $WORD is expanded and fed to the stdin of COMMAND.
As a simple example, consider this alternative to the echo-grep construction.
# Instead of:
if echo "$VAR" | grep -q txt
# if [[ $VAR = *txt* ]]
# etc.
# Try:
if grep -q "txt" <<< "$VAR"
then
echo "$VAR contains the substring sequence \"txt\""
fi
Or, in combination with read:
String="This is a string of words."
read -r -a Words <<< "$String"
# The -a option to "read"
#+ assigns the resulting values to successive members of an array.
echo "First word in String is: ${Words[0]}" # This
echo "Second word in String is: ${Words[1]}" # is
echo "Third word in String is: ${Words[2]}" # a
echo "Fourth word in String is: ${Words[3]}" # string
echo "Fifth word in String is: ${Words[4]}" # of
echo "Sixth word in String is: ${Words[5]}" # words.
echo "Seventh word in String is: ${Words[6]}" # (null)
# Past end of $String.
How-to: Here Documents
Here Strings - Advanced Bash-Scripting Guide.
macOS How To