Can someone explain this Task# and what does 1,4,5... mean?

In each CodeChef problem, there are certain ‘tasks’ (or Test cases, if you may), which contain different inputs under which your program is tested.

In Long Challenges, however, there are sub-tasks which reward you points for partially correct solutions (like the one you cooked).

Can someone explain this Task#

The task number of the test case input your program is fed with.

and what does 1,4,5… mean?

Task number.

So test cases 0,2,3 are working for my code which are under subtask 1 and the rest which are under the subtask 2 are not?
Thanks for the help.

Yes exactly. They might be working but you got a TLE so you have to work on your algorithm.