A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 816816 can be evenly divided by 1 2 3 4 6 7 8 11 12 13 14 16 17 21 22 24 26 28 33 34 39 42 44 48 51 52 56 66 68 77 78 84 88 91 102 104 112 119 132 136 143 154 156 168 176 182 187 204 208 221 231 238 264 272 273 286 308 312 336 357 364 374 408 429 442 462 476 528 546 561 572 616 624 663 714 728 748 816 858 884 924 952 1,001 1,092 1,122 1,144 1,232 1,309 1,326 1,428 1,456 1,496 1,547 1,716 1,768 1,848 1,904 2,002 2,184 2,244 2,288 2,431 2,618 2,652 2,856 2,992 3,003 3,094 3,432 3,536 3,696 3,927 4,004 4,368 4,488 4,641 4,862 5,236 5,304 5,712 6,006 6,188 6,864 7,293 7,854 8,008 8,976 9,282 9,724 10,472 10,608 12,012 12,376 14,586 15,708 16,016 17,017 18,564 19,448 20,944 24,024 24,752 29,172 31,416 34,034 37,128 38,896 48,048 51,051 58,344 62,832 68,068 74,256 102,102 116,688 136,136 204,204 272,272 408,408 and 816,816, with no remainder.
Since 816,816 cannot be divided by just 1 and 816,816, it is not a prime number.