A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1252860 can be evenly divided by 1 2 3 4 5 6 7 10 12 14 15 19 20 21 28 30 35 38 42 57 60 70 76 84 95 105 114 133 140 157 190 210 228 266 285 314 380 399 420 471 532 570 628 665 785 798 942 1,099 1,140 1,330 1,570 1,596 1,884 1,995 2,198 2,355 2,660 2,983 3,140 3,297 3,990 4,396 4,710 5,495 5,966 6,594 7,980 8,949 9,420 10,990 11,932 13,188 14,915 16,485 17,898 20,881 21,980 29,830 32,970 35,796 41,762 44,745 59,660 62,643 65,940 83,524 89,490 104,405 125,286 178,980 208,810 250,572 313,215 417,620 626,430 and 1,252,860, with no remainder.
Since 1,252,860 cannot be divided by just 1 and 1,252,860, it is not a prime number.