How to use JavaScript reduce() method to add ranges between an interval

The program below is used to add the four ranges with the ranges array and display the result 139.

(20 - 1) + (11930 - 11904) + (12020 - 11931) + (6 - 1) = 139

However, I cannot quite understand the use of the start parameter and the zero (0) in the code.

Help me understand how they are processed in the program.

Thanks

let ranges = [[1,20],[11904, 11930],[11931, 12020],[1,6]];
     console.log(ranges.reduce((start, [from, to]) =>{
       return start + (to - from);
     }, 0));