A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 621984 can be evenly divided by 1 2 3 4 6 8 11 12 16 19 22 24 31 32 33 38 44 48 57 62 66 76 88 93 96 114 124 132 152 176 186 209 228 248 264 304 341 352 372 418 456 496 528 589 608 627 682 744 836 912 992 1,023 1,056 1,178 1,254 1,364 1,488 1,672 1,767 1,824 2,046 2,356 2,508 2,728 2,976 3,344 3,534 4,092 4,712 5,016 5,456 6,479 6,688 7,068 8,184 9,424 10,032 10,912 12,958 14,136 16,368 18,848 19,437 20,064 25,916 28,272 32,736 38,874 51,832 56,544 77,748 103,664 155,496 207,328 310,992 and 621,984, with no remainder.
Since 621,984 cannot be divided by just 1 and 621,984, it is not a prime number.