Tethering is the process of sharing the mobile internet connection with other devices. For e.g. you can share your mobile internet connection with your tablet via WiFi.
If you are tethering via WiFi, then your tablet will consider it as a normal WiFi connection and it can consume large amount of data which can be costly.
In the Jelly Bean Android version, you can designate certain WiFi networks as mobile hot spots. Thus, even though it is a WiFi connection, the system will treat it as a mobile internet connection. You can then control the internet usage by disabling background services etc.
You can also set apps to warn you before using these networks for large download.
To mark certain WiFi networks as mobile hotspots, go to Settings > Wireless & Networks > Data Usage > Mobile Hotspots