What does <<>> mean in Erlang code?

Question

In some code I have seen something like this

{ok, Req2} = cowboy_req:reply(200, [], <<"Hello World!">>, Req),

I can't find out what does those <<>> mean.. Thank you.

Answer 1

In Erlang in they refer to binaries/bitstrings, see Bit Strings and Binaries and Bit Syntax Expressions.

Answer 2

This is for creating binary sequences. See relevant documentation.