A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 654588 can be evenly divided by 1 2 3 4 6 9 11 12 18 19 22 27 29 33 36 38 44 54 57 58 66 76 87 99 108 114 116 132 171 174 198 209 228 261 297 319 342 348 396 418 513 522 551 594 627 638 684 783 836 957 1,026 1,044 1,102 1,188 1,254 1,276 1,566 1,653 1,881 1,914 2,052 2,204 2,508 2,871 3,132 3,306 3,762 3,828 4,959 5,643 5,742 6,061 6,612 7,524 8,613 9,918 11,286 11,484 12,122 14,877 17,226 18,183 19,836 22,572 24,244 29,754 34,452 36,366 54,549 59,508 72,732 109,098 163,647 218,196 327,294 and 654,588, with no remainder.
Since 654,588 cannot be divided by just 1 and 654,588, it is not a prime number.