MBTiles is a file format for storing tilesets. It’s designed so that you can package the potentially thousands of files that make up a tileset and move them around, eventually uploading to Mapbox or using in a web or mobile application. MBTiles is an open specification and is based on the SQLite database. MBTiles can contain raster or vector tilesets.
You can export Mapbox Studio Classic sources in MBTiles format to upload directly to Mapbox Studio. See the Mapbox Studio manual uploads page for details. Below is a sample MBTiles file for reference.