Headers-first is a download process for initializing a blockchain using only the block headers. It is achieved by allowing for a more efficient and faster download of blocks from the full node by downloading block headers first, which use much less data than the full blocks.
This enables a full node to sync with a new version of the blockchain without having to download the entire blockchain. Once the headers have been downloaded, the node can then validate transactions and the chain without downloading the entire block.
This is achieved by downloading only block headers instead of full blocks that contain all of the transactions associated with the headers. By doing so, the node can quickly synchronize and remain up to date with any changes to the chain as new blocks are added.
Overall, the headers-first approach is an efficient way of downloading blockchain data while improving the download process and resource usage. It is a step forward in the goal of making blockchain technology more accessible and user friendly, while still providing a reliable and secure way to download blockchain data.