As you have undoubtely already discovered, poEdit's main window consists of three parts: there's a list of string-traslation pairs in the list in upper part of the window and two text entries under it. The first of them hold original string and is not editable, while the other is where you type your translations in. The list is separated from text fields with a splitter sash that you can move up and down to adjust percentage of screen covered by list and input fields.
There are two UI navigation modes built in poEdit and they differ in keyboard navigation handling. First of them (the default) leaves focus in the list, while the other redirects it to text input field. In the first mode, the list behaves like an ordinary list control and you have to press Tab to change focus to the text field before you can type in translations and press it once again to bring focus back to the list and move to another translation. This is done automatically in the second mode -- its drawback is that you can no longer use arrows for list navigation and has to use an alternative method described bellow. The 2nd mode is enabled by checking Always change focus to text input field in Preferences.
When the focus is in text field, you can use Ctrl-Up/Down arrows and Ctrl-PgUp/PgDown keys to move up and down the list list of messages. This works in both modes; it is the only keyboard navigation that works in the second mode (because the list never gets focus, you naturally can't use arrows without Ctrl modifier in this mode). This feature is particularly useful in the first mode where it eliminates the need to repeatedly press Tab.
You will see that some items have different color than others in the list. This is meant to simplify navigation in the catalog -- untranslated strings are marked with blue color and are always at the beginning of the list. Yellow color is used to mark fuzzy translations (see poEdit and Gettext Basics if you don't know what it means). You will notice that `not translated' and `fuzzy' marking is changed when edit translation, so you needn't worry about it in most cases.
Sometimes, there's a small icon to the left of entry in the list. There are currently three kinds of icons: a small green dot that indicates you modified the entry in this editing session, a pencil icon which means there's a comment attached to the entry (see menu description) and finally a gray picture of computer that is only shown if you updated the catalog and had configured poEdit to automatically roughly translated new entries (see settings and Translation Memory section).
You can right-click on focused item to display list of references (i.e. places in source code where given string appears) and open the file that contains given string. This is useful if you are not sure in what context is the string used. Same functionality is available via Edit/Show references menu command. The same popup menu will display suggested automatic translations if this feature is enabled (see Translation Memory section).
See Menu and Toolbar for description of toolbar icons.