Modes are switchable settings that enable or disable the operation of certain aspects of the software.
There are 4 mode commands on the Settings and Measurement tabs of the Ribbon.
They are:
Add/Deduct - set whether the next measure is going to be an Add or a Deduct
Notes - set whether Notes are automatically prompted for
Zero Height - set whether the height of picked points is forced to 0
Repeat - set whether the last measurement command is automatically repeated
Each is a variable setting to aid the measurement process and comes into affect on the next measurement.