The central pane is where you can visualize your NoSQL database model and collection schema.  The central pane adapts to the selected top and bottom tabs, for example:

Entity Relationship diagram of a database model:

With large models, it is possible that to fit the entire ERD on screen, the zoom level does not allow to read the information.  For performance reasons, Hackolade displays a sketchy rendition of the diagram:

You may control the the zoom level range of the sketchy rendition via a parameter in Tools > Options > General.

Hierarchical schema view of an entity schema:

JSON Preview of an entity:

Standard is limited to purely JSON Schema data types and keywords.

Full is limited to JSON Schema data types, but also includes all properties, including user-defined.

Extended generates an output that may include other data types than JSON Schema-compliant data types.

Referenced definitions lists $ref references to internal, model, and external definitions

Resolved definitions replaces the references by an instance of the respective definitions

A set of tabs appears at the top of the central pane.  They let you work on several collections in parallel, or consult relevant information in other collections.

The first tab is fixed and cannot be closed.  It is the main tab of the model, showing the Entity Relationship diagram for the database model, and other database model-level information.  The other tabs are collection tabs.  They are opened when you want to see or maintain collection-level information.  They can be closed individually by pressing the x on the right of the tab.  To close all tabs, use the shortcut Ctrl+M.  If you want to close all tabs except the current one, press Ctrl+Shift+M.

Note that the application remembers the opened tabs of your model, if you save it and exit, so you handily retrieve your configuration when you open the model again.

If you open more tabs than the width of the central pane can hold, the tabs will shrink as needed.  If you hover your mouse cursor over a tab, a handy tooltip pops-up to display the full name:

A set of tabs also appears at the bottom of the central pane.  The bottom tabs are fixed but differ depending on whether you select the database model top tab, or a collection tab:

Database model bottom tabs:

Collection bottom tabs:

Each bottom tab provides a different view of the entity information.