"Tether", used in the context here, is putting a mobile phone into the mode it becomes a wifi "hotspot".
So where there is no usable/affordable wi fi, your phone can provide that.
It does this by consuming some of its Data allowance.
For watching TV etc, or things video, and very influenced by the quality of the video, standard, HD, 2K & 4K it needs lots of bandwidth, and consumes "lots" of data. With generous packages of data watching video this way is doable.
It could also be draining the phones power quite heavily, so you might need to consider using the phone coupled up to a power outlet.
There are devices that directly create a wifi hotspot from the mobile phone network, stand alone units, in which you place a data SIM card. These in the same way use the credit embedded or placed on that SIM card, using up data allowance and creating a wifi hotspot.
An example
LINK
A hotspot, is a local transmitter of wifi, which normally is password protected, so only those who know that password can sap the owners credit up.