A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1553160 can be evenly divided by 1 2 3 4 5 6 7 8 10 12 14 15 20 21 24 28 30 35 40 42 43 56 60 70 84 86 105 120 129 140 168 172 210 215 258 280 301 344 420 430 516 602 645 840 860 903 1,032 1,204 1,290 1,505 1,720 1,806 1,849 2,408 2,580 3,010 3,612 3,698 4,515 5,160 5,547 6,020 7,224 7,396 9,030 9,245 11,094 12,040 12,943 14,792 18,060 18,490 22,188 25,886 27,735 36,120 36,980 38,829 44,376 51,772 55,470 64,715 73,960 77,658 103,544 110,940 129,430 155,316 194,145 221,880 258,860 310,632 388,290 517,720 776,580 and 1,553,160, with no remainder.
Since 1,553,160 cannot be divided by just 1 and 1,553,160, it is not a prime number.