When you open a Zotero library or collection, you see the references in a short form. In most cases you see author(s), year, part of the title, journal, reference type. One way you can manage what you would like to see in the center pane is by clicking with the right-mouse-button on the Title-bar and select the fields to display.
You can select references by clicking on one. In the screen below this is done for 1 reference (the 5th from the top).
The selected reference is shown on the right and you will be able to edit the data or attach a file.
In the example above the following fields are shown:
