In R programming what do numbers (underlined on the photo) before nrow and ncol mean when we loop through matrices? Do they indicate the row and the column the loop starts with?

Additionally, why would we need to loop through matrices? How can it be useful for data analysis in bioinformatics, for example?

for (r in 1: nrow (matrix3)){ for (c in 1: nco1 (matrix3)) \{ print (matrix3 [r,c])