For slopes lower than 4 the overlap is 18 inches half the width of the roll.
Roof dry in.
What is dry in.
In short drying in a structure means that the roof has been protected with at least a layer of felt a product like tyvek housewrap has been installed along with all the windows and doors.
Roof dry in flashing and underlayment.
Drying in for better lack of a term is making a house impervious to the rain or short for dry inside.
In australia they actually call this stage the lock up while the interior trim work is called a fix.
The roof sheathing is installed and a roof covering permanent or suitable reliable temporary roof waterproof.