What is a torrent and how it is used to download videos from the internet ?

Torrent network itself is not illegal since it is basically just a peer to peer file sharing network which can be used legally to share files according to the copyright restrictions on those files.
A torrent is file that contains the metadata about the file and folders along with information about other about computers (trackers) that help in finding the location of other computers which have the same file so that they can form a swarm for efficient file distribution.
Anybody using Torrent can create a Torrent file to share their contact. It is necessary that there is at least some one serving that particular file so that the sharing can happen. That is why for files like movies and games, usually you will get good download speed because many people are sharing the files.

Torrent apps and websites are still working and can be used to share legitimate content like ISO's of Linux operating system, binaries and shareware software like Tor.

Sharing files which are copyrighted is illegal and can be banned in institutions like academic and research oriented ones. Some software with cracks or full version sharing is not allowed and legal actions could be taken by appropriate authorities.

Torrent is said to be a computer file that helps to connect with other networks or computers for the purpose of file sharing. You can download a torrent of an individual file easily from the internet. However, since this thing can pirate files, documents and videos, it is prohibited in some countries. But still many people and "technicians" use it because it's allows you to download anything for free. They do this because licensed software, movies and music are expensive to buy in the market.