An android application is packaged into zip archive with the apk extension. You can extract it by the following command:
cd apk
unzip app-release.apk
After extracting the apk you can find classes.dex file - Dalvik Executable file. The file contains Dalvik bytecode which Dalvik VM or Android Runtime (ART) executes (translates to native instructions). Dalvik bytecode is translated Java bytecode. Thus if you want to read the source code of the app you have to convert dex file to java bytecode, then decompile java bytecode to java. To convert dex file to java bytecode (to jar - java archive with java class files) we will use dex2jar. You can download it from tools/ directory.