bootstrap.dat is a snapshot of the blockchain which can be used to bootstrap a new Bitcoin install.
In 2012, the Bitcoin blockchain was growing to 10+ gigabytes and downloading the entire chain from the p2p network was taking an excessive amount of time (multiple days) due to inefficiencies in the way nodes shared block data. In order make it easier to setup a full node, a snapshot of the blockchain data was distributed by Jeff Garzik as a file named bootstrap.dat using BitTorrent. Starting with version 0.8 bitcoind will load and validate initial blockchain data from any file named bootstrap.dat found in its data directory.
Today, this is actually slower than syncing normally. In Bitcoin Core version 0.10.0+ the p2p sync process was dramatically improved making it faster than using the bootstrap.dat file. This bootstrapping process is therefore obsolete and discouraged.
