A system present in all sharded blockchains that allows some data to be securely stored collaboratively among the validators. This is often used in to store Blocks produced in the L2 in the L1, so that thye can be reconstructed by the L1 Validators later for further inspection. JAM is the first blockchain that is attempting to use this technology for broader use-cases.
The Data Availability is a secure and short term storage; data stored in the L1 is often available for a fixed period of time (2-4 weeks), and it no history is kept for it.