Concept of APK

If you make an Android app called myapp. Then full name will be myapp.apk.
apk means application package.

apk package contains your code as an executable file along with images,sounds,etc. used by your app.

Executable file in Windows is .exe file like myapp.exe.
In Android mobile, executable file is .dex where dex stands for dalvik executable.

Dalvik is a Java virtual machine (JVM) made for Android mobile. JVM can understand .class file which is made by Java compiler javac.

javac can compile a .Java file written in Java language to .class file.

There is a software manager or app manager and launcher program continuously running in background as a service on Android operating system installed over a smartphone mobile.

This program on receiving an apk file of android app will search .dex file for executing its code using other files kept in its packaged apk file which is actually a compressed zip file. The app runs on being launched by a user click on its icon being displayed on mobile screen. This user click or touch signifies a user intent to launch this app from main thread of user interface app manager program of android OS in mobile.