Say I'm using a rapier (18-20=critical hit) and I attack someone and the d20 shows 17. If I use inspiration to add 1d6 to the roll does that make the die 18+ and thus a critical hit.
When you make an attack roll and get a natural 20 (the d20 shows 20), you hit regardless of your target's Armor Class, and you have scored a "threat," meaning the hit might be a critical hit (or "crit"). To find out if it's a critical hit, you immediately make an attempt to "confirm" the critical hit—another attack roll with all the same modifiers as the attack roll you just made. If the confirmation roll also results in a hit against the target's AC, your original hit is a critical hit. (The critical roll just needs to hit to give you a crit, it doesn't need to come up 20 again.) If the confirmation roll is a miss, then your hit is just a regular hit.
A critical hit means that you roll your damage more than once, with all your usual bonuses, and add the rolls together. Unless otherwise specified, the threat range for a critical hit on an attack roll is 20, and the multiplier is ×2.
Exception: Precision damage (such as from a rogue's sneak attack class feature) and additional damage dice from special weapon qualities (such as flaming) are not multiplied when you score a critical hit.
Increased Threat Range
Sometimes your threat range is greater than 20. That is, you can score a threat on a lower number. In such cases, a roll of lower than 20 is not an automatic hit. Any attack roll that doesn't result in a hit is not a threat.
Increased Critical Multiplier
Some weapons deal better than double damage on a critical hit (see Equipment).
Spells and Critical Hits
A spell that requires an attack roll can score a critical hit. A spell attack that requires no attack roll cannot score a critical hit. If a spell causes ability damage or drain (see Special Abilities), the damage or drain is doubled on a critical hit.
An investigator is beyond knowledgeable and skilled—he also possesses keen powers of observation and deduction that far surpass the abilities of others. An investigator typically uses these powers to aid in their investigations, but can also use these flashes of inspiration in other situations.
An investigator has the ability to augment skill checks and ability checks through his brilliant inspiration. The investigator has an inspiration pool equal to 1/2 his investigator level + his Intelligence modifier (minimum 1). An investigator's inspiration pool refreshes each day, typically after he gets a restful night's sleep. As a free action, he can expend one use of inspiration from his pool to add 1d6 to the result of that check, including any on which he takes 10 or 20. This choice is made after the check is rolled and before the results are revealed. An investigator can only use inspiration once per check or roll. The investigator can use inspiration on any Knowledge, Linguistics, or Spellcraft skill checks without expending a use of inspiration, provided he's trained in the skill.
Inspiration can also be used on attack rolls and saving throws, at the cost of expending two uses of inspiration each time from the investigator's pool. In the case of saving throws, using inspiration is an immediate action rather than a free action.
For- The result of the check is 17 and you add 1d6 to that.
Against- Natural means natural. Adding to the attack roll may help you hit the target but can not help you score a critical hit.