A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 7877408 can be evenly divided by 1 2 4 7 8 11 14 16 22 23 28 32 44 46 56 77 88 92 112 139 154 161 176 184 224 253 278 308 322 352 368 506 556 616 644 736 973 1,012 1,112 1,232 1,288 1,529 1,771 1,946 2,024 2,224 2,464 2,576 3,058 3,197 3,542 3,892 4,048 4,448 5,152 6,116 6,394 7,084 7,784 8,096 10,703 12,232 12,788 14,168 15,568 21,406 22,379 24,464 25,576 28,336 31,136 35,167 42,812 44,758 48,928 51,152 56,672 70,334 85,624 89,516 102,304 140,668 171,248 179,032 246,169 281,336 342,496 358,064 492,338 562,672 716,128 984,676 1,125,344 1,969,352 3,938,704 and 7,877,408, with no remainder.
Since 7,877,408 cannot be divided by just 1 and 7,877,408, it is not a prime number.