What is "web seed"?

According to the wikipedia page:

Web seeding was implemented in 2006 as the ability of BitTorrent clients to download torrent pieces from an HTTP source in addition to the swarm. The advantage of this feature is that a website may distribute a torrent for a particular file or batch of files and make those files available for download from that same web server; this can simplify long-term seeding and load balancing through the use of existing, cheap, web hosting setups. In theory, this would make using BitTorrent almost as easy for a web publisher as creating a direct HTTP download. In addition, it would allow the "web seed" to be disabled if the swarm becomes too popular while still allowing the file to be readily available.

As for the safety aspect, I would presume it is of no higher risk than normal torrent.

While normal seeds are other peer to peer users, a webseed is hosted on a server. Webseeds are used to guarantee the availability of the download, even if no other p2p users are sharing it.