Android ActionBar API, getSupportActionBar() and setDisplayHomeAsUpEnable meaning? -

i added icon on action bar. , when click on it, opens activity via intent. however, in source, people add getsupportactionbar , setdisplayhomeasupenable. if without those, still works.

my question meaning og these 2 api?

if have included actionbar or toolbar in activity , if want go previous activity on tap of back arrow on left side (in ltr configuration) of actionbar, have first actionbar as,

actionbar ab = getsupportactionbar(); 

and provide action go previous activity , have call setdisplayhomeasupenabled() method as,

if (ab != null) {     ab.setdisplayhomeasupenabled(true); }