Milestones help us to accurately determine whether a project is on schedule. When we add a Milestone to our chart we have a better insight into our project 's key dates.
When any Task (which is not a parent to another Task) is converted into a Milestone, it is placed on the Gantt chart in form of a rectangular - diamond-shaped figure, in the position of that Task's End Date (its both fields for Start and End Dates will now have its End Date's value).

In order to convert a task into a Milestone, we have to highlight our Task on the Gantt's WBS and select the 'Ellipsis' (three dots vector icon) menu to the right of our task and select 'Task Type' → 'Milestone'. We can convert Milestone to its original form by selecting a 'Regular Task' mode from the same menu.