Z order in xml android

can someone give me a good explanation regarding Z order. Firstly, I don't under开发者_开发百科stand what it is and(something like the definition)and secondly how an element of a Z order looks like. I hope I am explicit enough.

Z order determines how objects lay on each other - that is which one is on top. This is good example for HTML (with example pictures). In android is simpler (that is in my opinion conception is simpler, but usage could be less easy) since there is no explicit z index. If you use xml layout z order is determined by order of declaration views declaration in xml but if you prefer you can also change order from java code.

Starting on Android API level 21 the Z order is also affected by the android:elevation for a more detailed explanation see my answer here https://stackoverflow.com/a/38536318/2168700