A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 991872 can be evenly divided by 1 2 3 4 6 7 8 9 12 14 16 18 21 24 27 28 32 36 41 42 48 54 56 63 64 72 82 84 96 108 112 123 126 128 144 164 168 189 192 216 224 246 252 287 288 328 336 369 378 384 432 448 492 504 574 576 656 672 738 756 861 864 896 984 1,008 1,107 1,148 1,152 1,312 1,344 1,476 1,512 1,722 1,728 1,968 2,016 2,214 2,296 2,583 2,624 2,688 2,952 3,024 3,444 3,456 3,936 4,032 4,428 4,592 5,166 5,248 5,904 6,048 6,888 7,749 7,872 8,064 8,856 9,184 10,332 11,808 12,096 13,776 15,498 15,744 17,712 18,368 20,664 23,616 24,192 27,552 30,996 35,424 36,736 41,328 47,232 55,104 61,992 70,848 82,656 110,208 123,984 141,696 165,312 247,968 330,624 495,936 and 991,872, with no remainder.
Since 991,872 cannot be divided by just 1 and 991,872, it is not a prime number.