A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1378620 can be evenly divided by 1 2 3 4 5 6 9 10 12 15 18 20 23 27 30 36 37 45 46 54 60 69 74 81 90 92 108 111 115 135 138 148 162 180 185 207 222 230 270 276 324 333 345 370 405 414 444 460 540 555 621 666 690 740 810 828 851 999 1,035 1,110 1,242 1,332 1,380 1,620 1,665 1,702 1,863 1,998 2,070 2,220 2,484 2,553 2,997 3,105 3,330 3,404 3,726 3,996 4,140 4,255 4,995 5,106 5,994 6,210 6,660 7,452 7,659 8,510 9,315 9,990 10,212 11,988 12,420 12,765 14,985 15,318 17,020 18,630 19,980 22,977 25,530 29,970 30,636 37,260 38,295 45,954 51,060 59,940 68,931 76,590 91,908 114,885 137,862 153,180 229,770 275,724 344,655 459,540 689,310 and 1,378,620, with no remainder.
Since 1,378,620 cannot be divided by just 1 and 1,378,620, it is not a prime number.