By default, the main window contains six parts. The upper-right box is read-only and contains the current msgid (source text) field from the opened PO-file. The edit box just below this contains the msgstr (target text) field related to the msgid shown and here you can input or edit the translated text.
The top-left part of the main window shows the Translation Units. Below this, there is Unit Metadata section and it contains comments relevant to the currently displayed source text. In the lower-left, there is a Translation Memory section which shows the suggested translations from the translation memory database for the current source text entry. On the lower-right corner of the window, the glossary section is shown.
Translation files are opened in separate tabs, with two big multi-line edits as well as a bunch of tool views. These views can be stacked (similar to tabs), shown separately, or hidden. Translation files consist of many English-target pairs called units. A unit typically correspond to a single string in the user interface, or one paragraph in the documentation. The purpose of the first multi-line edit is to display the original part of the pair. The purpose of the second multi-line edit is to display the translation. You can navigate through the units via the Translation Units view or by using Page Down and Page Up.
A unit may be translated or untranslated. A translation of a translated unit may be ready or not ready (also called fuzzy sometimes). If the unit is not ready, its translation is rendered in italics. Lokalize allows you to easily navigate through the file according to the state of their translation. See Go menu for the shortcuts. The status bar at the bottom of the window shows the current string number, total number of strings, total untranslated strings, total not ready (fuzzy) strings and status of the current string respectively. When navigating, untranslated units are treated as not ready. Also, you may use the filtering feature of Translation Units toolview. Pressing Page Down actually takes you to the next unit in filtered/sorted list of that tool view.
To the main window one can add many more sections like Alternate Translations, Primary Sync, Secondary Sync, Binary Units by using → from the main menu.