What is meant by reserve day?
From Wikipedia - Limited overs cricket:
Important one-day matches, international and domestic, often have two days set aside, the second day being a "reserve" day to allow more chance of the game being completed if a result is not possible on the first day (for instance if play is prevented or interrupted by rain).
And why ICC not following this for even some big matches? In what basis this rule will applicable?
This depends on ICC and (if it's not a big tournament e.g. Tri-series etc) Cricket boards of playing countries. The reserve days are scheduled while scheduling the timetable of any tournament. For big tournaments numbers of days of the tournament are fixed as per their yearly schedule. And from the given days they schedule reserve days specially for latter stages (e.g. Quarter final, Semi final etc..). So that the match can be played again/resumed if the first day is interrupted by rain.