Process substitution can be used to create a file descriptor from the evaluation of a shell command. The syntax for process substitution is <(LIST) where LIST is one or more bash commands.

$ cat <(echo 'hello, world')
hello, world

This is particularly useful for commands that expect files, such as diff:

$ diff <(echo 'hello, world') <(echo 'hello, mars')
1c1
< hello, world
---
> hello, mars

Sources: Brian Dunn and Bash Guide for Beginners