You can download MyWi from Cydia using one of the following sources:
sinfuliphonerepo.com or repo.beyouriphone.com
What is Tethering?
It allows you to connect your laptop (or other types of devices) to the Internet through your cellphone in a place and at a time where and when your laptop may not be able to connect to the Internet or able to do so economically. It allows you to do many things beyond the limitations of a small cellphone.