Parallel vs Series
A Simple explanation is you can either process one request at a time or multiple. For example a For Each loop does one at a time where a job can run thousands at a time and can greatly reduce the amount of time a PowerShell script takes to execute. These are usually applicable when dealing with large quantities of data or computer / users. There are multiple CMDLets to assist in running things as Jobs / Runspaces etc…
These are a bit advanced concepts so don't spend tons of time on these if you are just beginning.
Here is a site with some more info: