If you're a developer working on translating an app, you most likely internationalized your code in preparation for localization. With pseudolocalization, you can verify and test the result of this process before translation begins. Once you load your source language files, Transifex can run a "fake" translation for you for free.
This "pseudo" translation, has mock-translation of phrases. You can load that file and render your app to detect various issues with your code. Here are some examples.
If you see phrases that are in English, you can tell that those phrases were not marked for translation or which were not included in the translation files.
If you see phrases in weird characters which should be in English (eg. your app name), those phrases were marked for translation but shouldn't have.
Pseudo files include Asian glyphs so you can detect problems with your rendering in Asian languages and detect rendered phrases that are too tall and are cropped.
The pseudo file makes phrases longer by default so you can identify rendered phrases that are too long and wrap them on your display. This is especially useful for languages that typically have longer words than English, such as German.
To download a pseudolocalized file, select a Project, click Languages, choose your source language (most likely English) and choose the resource for which you want the pseudo-file. In the popup, click "Get pseudo translations". You can now integrate this file in your app in development mode, launch it "localized" in this fake language, and check your work.