What does “<<<” mean on a bash command line? And where is it documented?

Question

What does <<< mean in this command line?

bc <<< \"1 + 1\"

It doesn\'t seem to a combination of <

Answer 1

It introduces a here string, documented near the end of the section on input and output redirections. A here string is just a one-word here document:

bc <<< "1 + 1"

is equivalent to

bc <