The stair ledger is the the part that is nailed to the wall or landing and used to fasten the stringers to. It can be one piece of wood or a couple as long as you get the right width to make the proper connection.
As you can see in the picture above their is not enough of the stair ledger to cover the stringer and this could result in the stringer splitting.
You can use 2 - 2 x 6 for a stair ledger.
Stringer to Ledger Nailing
Stair Ledger to Wall Nailing
Stair Tread Nailing
4 x 12 Stinger Base Problem