A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1050420 can be evenly divided by 1 2 3 4 5 6 7 10 12 14 15 20 21 28 30 35 41 42 60 61 70 82 84 105 122 123 140 164 183 205 210 244 246 287 305 366 410 420 427 492 574 610 615 732 820 854 861 915 1,148 1,220 1,230 1,281 1,435 1,708 1,722 1,830 2,135 2,460 2,501 2,562 2,870 3,444 3,660 4,270 4,305 5,002 5,124 5,740 6,405 7,503 8,540 8,610 10,004 12,505 12,810 15,006 17,220 17,507 25,010 25,620 30,012 35,014 37,515 50,020 52,521 70,028 75,030 87,535 105,042 150,060 175,070 210,084 262,605 350,140 525,210 and 1,050,420, with no remainder.
Since 1,050,420 cannot be divided by just 1 and 1,050,420, it is not a prime number.