How To make High Quality Textures Without Take A lot Of Size ?

Hi
now i am about to finish my game for google play store
but the textures quality is so low
when i make it high or true color
the size become so huge

and i saw a lot of games on the market has high quality
and the same time has low size for download
so i want to do the same
so what is the concept of this ?

 

Use compression, and design the textures to look okay compressed.

http://answers.unity3d.com/questions/786687/how-to-make-high-quality-textures-with-low-size-of.html

 

ok
i did that
and un check the override for android
but now
i want to know
what is compression do exactly ?

 

http://docs.unity3d.com/Manual/ReducingFilesize.html