A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1873920 can be evenly divided by 1 2 3 4 5 6 8 10 12 15 16 20 24 30 32 40 48 60 61 64 80 96 120 122 128 160 183 192 240 244 256 305 320 366 384 480 488 512 610 640 732 768 915 960 976 1,024 1,220 1,280 1,464 1,536 1,830 1,920 1,952 2,048 2,440 2,560 2,928 3,072 3,660 3,840 3,904 4,880 5,120 5,856 6,144 7,320 7,680 7,808 9,760 10,240 11,712 14,640 15,360 15,616 19,520 23,424 29,280 30,720 31,232 39,040 46,848 58,560 62,464 78,080 93,696 117,120 124,928 156,160 187,392 234,240 312,320 374,784 468,480 624,640 936,960 and 1,873,920, with no remainder.
Since 1,873,920 cannot be divided by just 1 and 1,873,920, it is not a prime number.