what does "<()" mean in bash?

I'm trying to install RVM. There is a magical command line:

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

I know what bash and curl are. I know the first < is the I/O redirection. But what does <() syntax mean?

What's the difference between this command and

bash < `curl -s https://rvm.io/install/rvm`

?(the latter command doesn't work)