A milestone is generally considered to be a key event, typically a deliverable. Milestones often indicate the beginning or ending of major phases or the completion of deliverable(s). Milestones are also excellent reporting points. 

• You can add a milestone task from the Add option on the Interactive Gantt or by setting the duration of the task as 0d.
• When you assign resources to this task, the allocation will automatically be pre-filled by 0%.
•  A milestone has 0 duration and as such has no additional calculation effect on the schedule. 
• A milestone can be linked to other tasks. 
• You can set constraint dates on a milestone.
•  These tasks are not shown on the Resource Load Chart.