Hi Yang Shiting,
The association from screenshot is called Cross module association.
Which means connecting 2 entities of different modules through an association .
From the screenshot – The association Visitor_Account is connection Visitor entity and Account entity which is there in another module.
Inorder to visit to other end – Right click on the association you will be seeing a popup which say “Go to other side”(below image reference).
When selected – you will be navigated to the connecting entity
Hope it helps!