A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1591590 can be evenly divided by 1 2 3 5 6 7 10 11 13 14 15 21 22 26 30 33 35 39 42 53 55 65 66 70 77 78 91 105 106 110 130 143 154 159 165 182 195 210 231 265 273 286 318 330 371 385 390 429 455 462 530 546 583 689 715 742 770 795 858 910 1,001 1,113 1,155 1,166 1,365 1,378 1,430 1,590 1,749 1,855 2,002 2,067 2,145 2,226 2,310 2,730 2,915 3,003 3,445 3,498 3,710 4,081 4,134 4,290 4,823 5,005 5,565 5,830 6,006 6,890 7,579 8,162 8,745 9,646 10,010 10,335 11,130 12,243 14,469 15,015 15,158 17,490 20,405 20,670 22,737 24,115 24,486 28,938 30,030 37,895 40,810 45,474 48,230 53,053 61,215 72,345 75,790 106,106 113,685 122,430 144,690 159,159 227,370 265,265 318,318 530,530 795,795 and 1,591,590, with no remainder.
Since 1,591,590 cannot be divided by just 1 and 1,591,590, it is not a prime number.