A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2456874 can be evenly divided by 1 2 3 6 7 9 14 17 18 21 31 34 37 42 51 62 63 74 93 102 111 119 126 153 186 217 222 238 259 279 306 333 357 434 518 527 558 629 651 666 714 777 1,054 1,071 1,147 1,258 1,302 1,554 1,581 1,887 1,953 2,142 2,294 2,331 3,162 3,441 3,689 3,774 3,906 4,403 4,662 4,743 5,661 6,882 7,378 8,029 8,806 9,486 10,323 11,067 11,322 13,209 16,058 19,499 20,646 22,134 24,087 26,418 33,201 38,998 39,627 48,174 58,497 66,402 72,261 79,254 116,994 136,493 144,522 175,491 272,986 350,982 409,479 818,958 1,228,437 and 2,456,874, with no remainder.
Since 2,456,874 cannot be divided by just 1 and 2,456,874, it is not a prime number.