Portable object (.po) files are used by programs like WordPress to be able to offer the specific translation of a text in English to any other language when a dialog box appears, or in other words, it translates the onscreen answer that is seen by the user depending on the language settings that he/she has chosen.
The biggest problem of these files is that it isn't at all easy to edit them unless you have access to Poedit, a free open source program specifically designed for this purpose.
The interface used by Poedit is very clear, showing on the top half all the dialog lines with the original text on the left hand side and its translation on the right hand side. When you select any line, the corresponding text appears in the two text boxes that are in the bottom half of the interface, offering the possibility to edit them.
Among the most noteworthy features of Poedit, it's worth highlighting the option that marks the areas where there are two or more blank spaces and the possibility to automatically compile the corresponding .mo file when any file is saved.
Download Poedit to easily edit any PO file.