Question

explain the term 'standard meridian'

Answer 1

i thnk Standard Meridian is that the meridian used for reckoning standard time; throughout most of the world the standard meridians are those whose longitudes are exactly divisible by 15°. 

Answer 2

this meridian is used to reckon the standard time of a country. through most of the world
it is divisible by 15 degrees .
as 15 degrees longitude = 1 hour in time