A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1256736 can be evenly divided by 1 2 3 4 6 8 12 13 16 19 24 26 32 38 39 48 52 53 57 76 78 96 104 106 114 152 156 159 208 212 228 247 304 312 318 416 424 456 494 608 624 636 689 741 848 912 988 1,007 1,248 1,272 1,378 1,482 1,696 1,824 1,976 2,014 2,067 2,544 2,756 2,964 3,021 3,952 4,028 4,134 5,088 5,512 5,928 6,042 7,904 8,056 8,268 11,024 11,856 12,084 13,091 16,112 16,536 22,048 23,712 24,168 26,182 32,224 33,072 39,273 48,336 52,364 66,144 78,546 96,672 104,728 157,092 209,456 314,184 418,912 628,368 and 1,256,736, with no remainder.
Since 1,256,736 cannot be divided by just 1 and 1,256,736, it is not a prime number.