There is a << operator (here document) but it is not what is used here.

You have first an input redirection < which says pick the input from the following argument then there is the <(command) known as process substitution which says replace that by a file descriptor containing the output of of command.

This command could have been written

bash < <(curl -s https://rvm.beginrescueend.com/install/rvm)

but is just a convoluted way of doing:

curl -s https://rvm.beginrescueend.com/install/rvm | bash

There is a << operator (here document) but it is not what is used here.

You have first an input redirection < which says pick the input from the following argument then there is the <(command) known as process substitution which says replace that by a file descriptor containing the output of the command.

This command could have been written

bash < <(curl -s https://rvm.beginrescueend.com/install/rvm)

but is just a convoluted way of doing:

curl -s https://rvm.beginrescueend.com/install/rvm | bash