If a to Strike roll results in a 20 (before bonuses) when rolling 1D20 the result is called a Natural 20.
A strike with a Natural 20 will always be a Critical Strike. If someone with a Strike bonus of +2 rolls an 18, then the resulting 20 is not a Natural 20 (18+2 = calculated 20), this is not a Critical Strike. A Natural 20 beats all other rolls and can only be Parried or Dodged by another Natural 20.