What is bootstrap.dat and what does it do?
To speed up the process of having the complete and up to date blockchain, instead of having your wallet syncing it block by block, a file called bootstrap.dat file can be used. The bootstrap.dat file contains the latest blockchain information needed to have your wallet synced and up to date. By copying it to your XTRABYTES folder, the wallet will pick it up and import the content, reducing the time needed for syncing.

For complete details, visit our forum: https://community.xtrabytes.global/threads/sync-the-xtrabytes-wallet-faster-with-the-bootstrap-dat-file.434/#post-5431