The Manni are a religious group that is known for their experience travelling to other worlds. They worship things called "The Over" and "The Force." The Over appears to be the force that Roland Deschain calls the Prim. They speak their own language that is mostly the same as English or the High Speech, but with a few differences.
Their appearance is similar to the Quakers and the Amish in our world. The men wear long blue cloaks and long beards. They also have long fingernails as they are only allowed to cut them twice a year.
The Manni have a specific method for travelling between worlds. They use plumb bobs and magnets to help channel energy to allow for this travel. The senders are the people most skilled at channeling this energy. Being strong in "the touch" is also useful for travel.
The Johns family have a tradition started with Sir Alfred Johns where they marry the women of the Manni tribes and so their family is strong in 'the touch.'