This tool helps you prepare CSV spreadsheets for importing data into various types of record in CiviCRM. Full documentation, including a worked example is available in the Documentation.
What problems does this help with?
You have a load of data to import as contributions or activities (etc.) to import but because this came from an external source it doesn't have CiviCRM Contact IDs in it.
You have data to import to contacts but it's a bit of a mess and may result in duplicates, even despite your expertise in using CiviCRM's dedupe rules.
You have data to import but the name field is just one field instead of first and last names.
How does it help?
You feed it a CSV (Comma-Separated Values) and it uses an interactive process to help you identify the contacts for each row. You can also have it create contacts when you have decided they need to be.
It tries to be helpful by looking for matches for you, based on name and email. If it finds a single matching contact it will auto-select that, otherwise it will offer you options, e.g. "Same email: Wilma Flintstone", or "Similar name: Betty Rubble".
Also it only shows you distinct sets of first, last names and emails. So if your data has a bunch of records from Wilma Flintstone, you'll only have to locate Wilma once.
When everything is matched up, you can download the CSV file again, which will be the same file as you fed into it, but now it has an extra "Internal ID" column with the CiviCRM Contact ID populated.
Then you can use that in CiviCRM's normal import routines for contacts, activities, contributions...