A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1020510 can be evenly divided by 1 2 3 5 6 9 10 15 17 18 23 29 30 34 45 46 51 58 69 85 87 90 102 115 138 145 153 170 174 207 230 255 261 290 306 345 391 414 435 493 510 522 667 690 765 782 870 986 1,035 1,173 1,305 1,334 1,479 1,530 1,955 2,001 2,070 2,346 2,465 2,610 2,958 3,335 3,519 3,910 4,002 4,437 4,930 5,865 6,003 6,670 7,038 7,395 8,874 10,005 11,339 11,730 12,006 14,790 17,595 20,010 22,185 22,678 30,015 34,017 35,190 44,370 56,695 60,030 68,034 102,051 113,390 170,085 204,102 340,170 510,255 and 1,020,510, with no remainder.
Since 1,020,510 cannot be divided by just 1 and 1,020,510, it is not a prime number.