As you'll read in a moment, it's about "persistent task". I couldn't find a smart name for that, so I kinda went phonetically.
This library is supposed to be used when you have long running tasks that depend on each other. Thanks to Percival you can define tasks that are run once and their results (as well as run mmetadata) are stored in a database.
Right now it's hardcoded to use MongoDB.
This can be particularly useful in scientific research in IT, ML, etc. You can write down your experiment plan, process the results and then decide what to do next. At that point you extend your code with next steps and rerun the app.
Initial part will be recognized as persisted and won't be executed. It's results will be available to new experiments, e.g. to derive parameters.
Besides, if you need to shut down the app, you can salvage results from previous run in next run.