It means it does triple damage to what it hits on a critical hit
Many traps are designed as weapons that automatically attack. So they use the same rules and notation as weapons that characters and monsters wield. Weapon notation in creature and trap entries includes five things:
- the base dice (the die or dice rolled on a successful hit to determine how much base damage is done; here, 1d8)
- the damage modifier (derived from Strength, magic weapons, feats, and other modifiers; here, +1, like a Strength +1 composite bow)
- any rider effects that trigger on a hit (such as poison, extra fire damage, and so on; here, there aren't any)
- the critical threat range (the range of raw numbers shown on the attack d20 that can trigger a critical; here, the default of 20, as it's not mentioned), and
- the critical multiplier (the total damage done on a confirmed critical hit; here, triple the normal).
When the trap activates, you roll a d20 to attack in the usual way; if the result of the d20 itself (before adding attack modifiers) is one of the numbers in the threat range (here, just 20), it's a critical threat, and you roll again to see if the second roll is a hit — if so, the original hit is a critical hit, otherwise it's just a regular hit. If it's confirmed as a critical, you roll the base dice and damage modifier more than once and add the totals together: here, three times, or 3d8+3. (You can also go the lazy route and just roll once and multiply by three, but that doesn't give exactly the same results.)
A critical hit doesn't attack any more creatures than would normally be possible, just does more damage to what it does hit.