A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1243788 can be evenly divided by 1 2 3 4 6 7 12 13 14 17 21 26 28 34 39 42 51 52 67 68 78 84 91 102 119 134 156 182 201 204 221 238 268 273 357 364 402 442 469 476 546 663 714 804 871 884 938 1,092 1,139 1,326 1,407 1,428 1,547 1,742 1,876 2,278 2,613 2,652 2,814 3,094 3,417 3,484 4,556 4,641 5,226 5,628 6,097 6,188 6,834 7,973 9,282 10,452 12,194 13,668 14,807 15,946 18,291 18,564 23,919 24,388 29,614 31,892 36,582 44,421 47,838 59,228 73,164 88,842 95,676 103,649 177,684 207,298 310,947 414,596 621,894 and 1,243,788, with no remainder.
Since 1,243,788 cannot be divided by just 1 and 1,243,788, it is not a prime number.