A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1230840 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 13 15 18 20 24 26 30 36 39 40 45 52 60 65 72 78 90 104 117 120 130 156 180 195 234 260 263 312 360 390 468 520 526 585 780 789 936 1,052 1,170 1,315 1,560 1,578 2,104 2,340 2,367 2,630 3,156 3,419 3,945 4,680 4,734 5,260 6,312 6,838 7,890 9,468 10,257 10,520 11,835 13,676 15,780 17,095 18,936 20,514 23,670 27,352 30,771 31,560 34,190 41,028 47,340 51,285 61,542 68,380 82,056 94,680 102,570 123,084 136,760 153,855 205,140 246,168 307,710 410,280 615,420 and 1,230,840, with no remainder.
Since 1,230,840 cannot be divided by just 1 and 1,230,840, it is not a prime number.