As part of Polkadot's availability system, certain pieces of data for each block are required to be kept available.
The way we accomplish this is by erasure coding the data into n pieces and constructing a merkle root of the data.
Each of n validators stores their piece of data. We assume n = 3f + k, 0 < k ≤ 3.
f is the maximum number of faulty validators in the system.
The data is coded so any f+1 chunks can be used to reconstruct the full data.