Process Substitution is a form or redirection where the input of output of a process appear as a temporary file.

<( <LIST> ) 

>( <LIST> )

Explanation:

The command LIST is executed and its:

• standard output filedescriptor in the <( … ) form or
• standard input filedescriptor in the >( … ) form

is connected to a FIFO or a file in /dev/fd/.

• Useful for redirecting output of a command to something that expects input from a file and not from stdin.

See: Bash Hackers Wiki - Process Substitution

Further explanation #

• Get variables out of subshells (and pipes) <(list) >(list)

read < <(echo "foo")
echo $REPLY

#!/bin/bash

# pro-sub: demo of process substitution

while read attr links owner group size date time filename; do
    cat <<- EOF
    Filename:   $filename
    Size:       $size
    Owner:      $owner
    Group:      $group
    Modified:   $date $time
    Links:      $links
    Attributes: $attr
    EOF
done < <(ls -l | tail -n +2)

Cautionary #

The scope of process substitution file descriptors is not stable, guaranteed, or specified by bash.

See: Bash Hackers Wiki - Process Substitution