In short, it comes from its CON modifier.
The numbers in brackets provide formula to roll for the monster's HP and it works just the same as with PCs. In this case the 15d8 comes from a medium (hence the d8; a small one would have a d6 and so on) monster on a 15th "level" (not really a normal level, just a concept used in building the stats) plus 15×[CON MOD]. So I would suspect that the monster has CON +5 (since 75 = 15×5).
You can then check that average HP is 15 × 4.5 + 75 = 142 (round down).
