In the “Relations” tab you can see and/or add relations between entities. You can see a description of the different types of relations here. Note that planning an item is done in Edit Details form, read more here.
The view shows all the types relations that can be added for this specific item. When a relation is created it can carry a description, which explains the purpose of this relation. An existing relation can be edited (to update the description) or deleted.
If you want to create a new relation from the current item, start typing the id or title of the item, to which you want to create a relation, or browse to find the item by clicking the folder icon.