Swift, also known as the OpenStack Object Store project, is the S3 compatible object storage service canonically associated with OpenStack deployments which in recent years essentially is simply an API, to interact with object storage in OpenStack as a common interface to storage backends like Ceph. It is software that allows for storage and retrieval of data via a simple API. It is scalable, and optimized for durability, availability, and concurrency. Swift is ideal for storing unconstrained, redundant data. Due to this being a service for object storage, Swift allows for an API accessible storage option which can be used for backups, data retention, or archives that is redundant across the cluster.
You can find and contribute to Swift documentation here.