"Femto" means a factor of 10-15, a thousandth of a millionth of a millionth. So a femtobarn is a barn (10-28 square metres) multiplied by 10-15, which equals 10-43 square metres.
When writing pages for the general public, always give an approximate number of collisions as well as the figure in inverse femtobarns.
One inverse femtobarn is equal to approximately 100 trillion proton-proton collisions.
Be sure to include the word "approximately" or "about" before the collision number you cite - the figure is a general conversion rather than a precise estimate.