A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1623840 can be evenly divided by 1 2 3 4 5 6 8 10 12 15 16 17 20 24 30 32 34 40 48 51 60 68 80 85 96 102 120 136 160 170 199 204 240 255 272 340 398 408 480 510 544 597 680 796 816 995 1,020 1,194 1,360 1,592 1,632 1,990 2,040 2,388 2,720 2,985 3,184 3,383 3,980 4,080 4,776 5,970 6,368 6,766 7,960 8,160 9,552 10,149 11,940 13,532 15,920 16,915 19,104 20,298 23,880 27,064 31,840 33,830 40,596 47,760 50,745 54,128 67,660 81,192 95,520 101,490 108,256 135,320 162,384 202,980 270,640 324,768 405,960 541,280 811,920 and 1,623,840, with no remainder.
Since 1,623,840 cannot be divided by just 1 and 1,623,840, it is not a prime number.