For <(1 , 3) , (2 , -1)> , <i , j> = 0 why 0 and what does the <i , j> mean?

This is the Range=distance of mark; set in a row; pass from one point to another; a range-finder. In Maths,it means (for negative)2--->-3= -1; (for positive)4--->1= 5; Hence the difference range-finder=(5-1=4=2-2=0; or 2=2!)
Hence (i, j)=0 means similarly the difference in the range-finder (i=j; or i-j=0!)