Your Splice folder is where Sounds you add to your Library are downloaded. The Splice desktop application accesses your Sounds from your Splice folder. If you don’t have the correct folder synced to your desktop app, you won’t be able to use some of the desktop app’s features, such as drag and drop.
When you download your first Sound from Splice, a folder will automatically get created in one of these locations, depending on which OS you're running:
MacOS: Macintosh HD/Users/[your username]/Splice
You're free to move this folder elsewhere on your computer or rename it. If you do either of those things, make sure you're setting the new location in the Preferences section of the Splice app after making any changes. To access your Preferences, click on the icon at the top right of the desktop app. To jump directly to your Splice folder in Finder or File Explorer, just click the "Go to folder" button in the Preferences of the Splice app.