Find the Mark (Ex): At 8th level, the swashbuckler
increases his threat range by one when using any
melee weapon with which he has Weapon Finesse.
A weapon that would threaten a critical on 20 would
now do so on a 19 or 20, and one which threatens
on a 19 or 20 would now do so on an 18 to 20.
This ability works with other abilities which increase
threat ranges.
So basically, if you had a rapier (18-20), it would become 15-20, and if you had a keen rapier or the improved critical feat, it would become 12-20, and then with improved find the mark at 16th level, it would become a 9-20 crit range.