The root node of the class hierarchy for the code in this directory is BucketContext. Start off by reading that class's documentation.

Each BucketContext and its owned objects are associated with a single IndexedDB instance, such as that accessed via navigator.indexedDB or the indexedDB for a storage bucket.

Code in this directory mostly runs on the SequencedTaskRunner that is created by the IndexedDBContextImpl for the BucketContext when the former spawns the latter. This is sometimes referred to as the “bucket thread”.

The only entry point into this directory that external code should access is the BucketContext, but classes in this directory can use some thread-agnostic code such as ../file_path_util.h.