A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 870408 can be evenly divided by 1 2 3 4 6 7 8 9 11 12 14 18 21 22 24 28 33 36 42 44 56 63 66 72 77 84 88 99 126 132 154 157 168 198 231 252 264 308 314 396 462 471 504 616 628 693 792 924 942 1,099 1,256 1,386 1,413 1,727 1,848 1,884 2,198 2,772 2,826 3,297 3,454 3,768 4,396 5,181 5,544 5,652 6,594 6,908 8,792 9,891 10,362 11,304 12,089 13,188 13,816 15,543 19,782 20,724 24,178 26,376 31,086 36,267 39,564 41,448 48,356 62,172 72,534 79,128 96,712 108,801 124,344 145,068 217,602 290,136 435,204 and 870,408, with no remainder.
Since 870,408 cannot be divided by just 1 and 870,408, it is not a prime number.