When we unzip a APK we get a compiled version of the application in DEX (Dalvik EXecutable), we want to create a human readable format, which is also called SMALI.
Or we can use tools like JADx to convert from APK to Java format that tries to simulate how the original code would look like.
With JADx we would be able to view source code but not modify it, while Apktools we would be able to modify SMALI code but it's harder to create therefor we won't be able to analyse it.