GNU Parallel is a shell tool for executing jobs in parallel using one or more machines. A job is typically a single command or a small script that has to be run for each of the lines in the input. The typical input is a list of files, a list of hosts, a list of users, or a list of tables.
If you use xargs today you will find GNU Parallel very easy to use. If you write loops in shell, you will find GNU Parallel may be able to replace most of the loops and make them run faster by running jobs in parallel. If you use ppss or pexec you will find GNU Parallel will often make the command easier to read.
GNU Parallel also makes sure output from the commands is the same output as you would get had you run the commands sequentially. This makes it possible to use output from GNU Parallel as input for other programs.