Predecessor

Some tasks have predecessors, which that are dependent on other tasks to be completed before they can be started. (When tasks can be carried out at the same time, they are considered to be concurrent.)

Example Project

To make coffee, your tasks might include:
1 - gather the tools you will need to prepare coffee
2 - heat the water
3 - grind the beans
4 - prepare the coffee pot
5 - brew the coffee
6 - serve the coffee
 
Clearly, Task #1 must happen before Task #5, 
so Task #1 is a predecessor to Task #5.

Tasks #2, #3 and #4 can happen in any order, or at the 
same time (if you have a person to help). These Tasks may 
be known as concurrent tasks.

Task #6 is always the last step, and can never be a 
predecessor in the coffee-making project.