A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 386880 can be evenly divided by 1 2 3 4 5 6 8 10 12 13 15 16 20 24 26 30 31 32 39 40 48 52 60 62 64 65 78 80 93 96 104 120 124 130 155 156 160 186 192 195 208 240 248 260 310 312 320 372 390 403 416 465 480 496 520 620 624 744 780 806 832 930 960 992 1,040 1,209 1,240 1,248 1,488 1,560 1,612 1,860 1,984 2,015 2,080 2,418 2,480 2,496 2,976 3,120 3,224 3,720 4,030 4,160 4,836 4,960 5,952 6,045 6,240 6,448 7,440 8,060 9,672 9,920 12,090 12,480 12,896 14,880 16,120 19,344 24,180 25,792 29,760 32,240 38,688 48,360 64,480 77,376 96,720 128,960 193,440 and 386,880, with no remainder.
Since 386,880 cannot be divided by just 1 and 386,880, it is not a prime number.