What is the real meaning of % and int arr(3)={5,3,1}.i'm really don't understand

Answer 1

% shows the remainder from the division. For example, 5%2=1

Answer 2

As Remmae said % operator gives you the remainder (% is called modulo division operator )
In C lang arr is a variable....
here this variable arr is declared as an integer array with a size of 5 (size = no. of elements)
where arr[0] = 5, arr[1] = 3, arr[2] = 1 >> the number in [ ] indicates the index of a number in that arrray