A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1881984 can be evenly divided by 1 2 3 4 6 8 12 13 16 24 26 29 32 39 48 52 58 64 78 87 96 104 116 128 156 169 174 192 208 232 312 338 348 377 384 416 464 507 624 676 696 754 832 928 1,014 1,131 1,248 1,352 1,392 1,508 1,664 1,856 2,028 2,262 2,496 2,704 2,784 3,016 3,712 4,056 4,524 4,901 4,992 5,408 5,568 6,032 8,112 9,048 9,802 10,816 11,136 12,064 14,703 16,224 18,096 19,604 21,632 24,128 29,406 32,448 36,192 39,208 48,256 58,812 64,896 72,384 78,416 117,624 144,768 156,832 235,248 313,664 470,496 627,328 940,992 and 1,881,984, with no remainder.
Since 1,881,984 cannot be divided by just 1 and 1,881,984, it is not a prime number.