When you roll you attack roll, you can score a "critical hit".
The number you need to roll to score a critical is shown in the shop table, right before the price.
i.e. if you have a short sword (crit:19-20/x2), you score a critical hit if your attack roll equals 19 or 20 (before you apply your attack bonus). If you score a critical hit with a short sword, the damage roll will be multiplicated by 2 (19-20/x2).
Please note that some weapon have other multiplicators. It is sometimes x3. If the critical info shows only x3, you score a critical only on an attack roll of 20.