Trello doesn't have a generic import tool, but you have several options for bringing your data into Trello:
Copy Paste - When you paste text into a Trello card, each new line becomes a new card. This means if you have a list of card titles e.g. in an Excel spreadsheet, you can simply copy the title from one column, and each cell from the sheet will become a card in a Trello list.
Trello API - You can use the Trello API to have full programmatic control of the import process
Third-Party Power-Ups - There are a number of third-party Power-Ups developed by people or companies external to Trello that are able to import data into Trello, you can search for a list of these and find one that's best for you here: https://trello.com/power-ups/search?q=import
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