Welcome to the ODK forum, @alinephilibert ! We're glad you're here. When you get a chance, please introduce yourself on this forum thread. I'd also encourage you to add a real picture as your avatar because it helps build community!
ODK Briefcase is a desktop application that runs on macOS, Windows, and Linux. It is used for pulling, pushing, and exporting forms on ODK servers such as ODK Central and ODK Aggregate. You can also use Briefcase to pull forms directly from ODK Collect.
Here's the documentation specifically on getting data from ODK Collect: https://docs.getodk.org/briefcase-using/#pull-from-collect
Can you read through that and let us know if you have specific questions about the process?