A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1657908 can be evenly divided by 1 2 3 4 6 7 9 12 14 17 18 21 27 28 34 36 42 43 51 54 63 68 81 84 86 102 108 119 126 129 153 162 172 189 204 238 252 258 301 306 324 357 378 387 459 476 516 567 602 612 714 731 756 774 903 918 1,071 1,134 1,161 1,204 1,377 1,428 1,462 1,548 1,806 1,836 2,142 2,193 2,268 2,322 2,709 2,754 2,924 3,213 3,483 3,612 4,284 4,386 4,644 5,117 5,418 5,508 6,426 6,579 6,966 8,127 8,772 9,639 10,234 10,836 12,852 13,158 13,932 15,351 16,254 19,278 19,737 20,468 24,381 26,316 30,702 32,508 38,556 39,474 46,053 48,762 59,211 61,404 78,948 92,106 97,524 118,422 138,159 184,212 236,844 276,318 414,477 552,636 828,954 and 1,657,908, with no remainder.
Since 1,657,908 cannot be divided by just 1 and 1,657,908, it is not a prime number.