Verily, this is a reward for you, and your endeavor has been accepted.

meaning, this will be said to them in honor of them and as a goodness towards them.

This is as Allah says,

كُلُواْ وَاشْرَبُواْ هَنِييَاً بِمَأ أَسْلَفْتُمْ فِى الاٌّيَّامِ الْخَالِيَةِ 


Eat and drink at ease for that which you have sent forth before you in days past! (69:24)

Allah also says,

وَنُودُواْ أَن تِلْكُمُ الْجَنَّةُ أُورِثْتُمُوهَا بِمَا كُنتُمْ تَعْمَلُونَ

And it will be cried out to them:"This is the Paradise which you have inherited for what you used to do." (7:43)

Then Allah says,

وَكَانَ سَعْيُكُم مَّشْكُورًا

and your endeavor has been accepted.

meaning, `Allah the Exalted will reward you for a small amount (of deeds) with a large amount (of reward).