The Units value you can edit in the Assign Resources dialog is an important part of scheduling your resources in Project. This value controls how Project schedules the Assignment Work value for the resource. A 100% Units value means that for the duration of the assignment, the resource will be scheduled to work 100% of the hours its calendar defines as working time.
The following example shows a five-day, Fixed Units task to which Brian has been assigned at 100%. Because Brian's calendar says that 100% means 8 hours per day, this makes the Assignment Work value equal to 40 hours.
If we edit Assignment Units to 50%, the Assignment Work value will stay the same, but the duration of the task will increase because the change to 50% means that each day will only contain four hours of work. Therefore, the duration must increase to accommodate the Work, as shown here.
This example covers how a Fixed Units or Fixed Work task would react to the editing of the Units value. A Fixed Duration task would have reduced the Assignment Work value. Duration was fixed at five days (Fixed Duration), so a decrease in Assignment Units would have to lower the Work value to keep the Duration = Work/Units formula in balance.
In general, you can think of Assignment units as a full-time / part-time measurement. A 100% assignment means that a resource is working what its calendar defines as full-time. A 50% assignment is half-time.