What does "Tethering" mean exactly?

[Monstrosity]

I hear about the data plan is good but you can't tether with it or it will charge extra or whatever.

What EXACTLY, is considered "tethering"?

 

[SoCal65]

Here's a quick answer...

Tethering is the use of a mobile device such as a mobile phone to supply Internet access for another device which is otherwise unconnected, using the connected device as a modem.

e.g. Plug your phone into your laptop so your laptop can surf the internet. Your phone becomes a modem.