While moving on bus, I already marked my destination and clicked "directions" But below there is option called "start". What does it do?
The "start" button will start the turn-by-turn driving directions mode (looks something like this) where it will show your position and highlight each upcoming turn or merge. If you have the sound on, it will also announce each turn for you out loud.
Unless you're actually driving the bus, this is not all that useful on public transit, as you won't need the directions and Google won't be following the preset bus route. Google Maps does have a transit directions feature (the train icon, second from the right), but it does not appear to be supported in your particular area (you can see a list of all cities with transit coverage).