These are snapshots of a clean VEIL installation synchronized from scratch.
They contain the folders "blocks", "chainstate", "indexes" and "zerocoin". New snapshots are created every three days at about 00.15 am.
You should make sure that you have enough bandwidth and storage for the full blockchain snapshot (currently over 18G).
In order to conserve the limited bandwidth available, we automatically choose a mirror site near you to download the daily blockchain snapshot.
We automatically poll the mirror list on a regular basis to ensure that sites have the latest blockchain snapshot available.
How to use a snapshot
See this support article: How to use a snapshot