Tethering is a term carrier's use when you turn your device into a wifi hotspot using the data included in your plan. Not all carriers allow this but the majority of all modern devices have the capability to be used as a mobile hotspot.
The only catch, it all depends on your service plan and data allowances. Not all plans allow tethering so you if you are interested in turning your device into a mobile hotspot you might want to upgrade your plan or switch carriers.
You need to make sure the device you've got or thinking of purchasing can actually be used as a hotspot. If tethering is important to you, then make sure you choose a plan that allows this.
I have included a search results page of all the plans that include tethering