The idea is to go through 10,000 sentences in a language you learn. First you will see a sentence in a language you known and then you must guess the words of the same sentence in your target language. By exposing you to unknown words in a real context (a sentence) you will slowly (but steadily) increase your vocabulary size.
This app is a helper tool to help you increase your vocabulary by exposing you to thousands of sentences. In addition to using it, you should also pursue other (more traditional) ways of learning a language: speaking to native speakers, learning grammar, etc.
You must have Google translate installed for this option to work.
If you use another translation app, you can it open with the "Open with..." option in the menus.
The application allows you to write annotations on words. More than one words can be annotated with the same annotation.
Annotations are best used to create your own dictionary as you proceed learning new sentences. If you use annotations like a dictionary, make sure that you write them in grammatically neutral way. For example, if you learn Italian, you will need to put "camminare" (to walk), "cammino" (I walk), "camminero" (I will walk) under one annotation (for example "walk").
This application is not able to discern if "camminero" (I will walk) and "camminavo" (I was walking) are one and the same word. It is up to you to figure out that it must be one annotation, but this journey of "figuring out" words is part of language learning.
Most Android phones have text-to-speech (TTS) available for some languages. Some languages have very poor TTS voice quality. Check your phone settings to see if there is an option to download a better quality TTS. Some languages have also additional TTS applications available on the Google Play store.