What is an inverse femtobarn?

“A barn (symbol b) is a unit of area. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is also used in all fields of high-energy physics to express the cross sections of any scattering process, and is best understood as a measure of the probability of interaction between small particles. A barn is defined as 10−28 m2 and is approximately the cross-sectional area of a uranium nucleus.” – Wikipedia

The cross-section of the uranium nucleus (1 barn) is $10^{-28} m^2$ and femto means $10^{-15}$, a thousandth of a millionth of a millionth.  So a femtobarn is $10^{-43} m^2$.

The inverse femtobarn gives you an estimate number of particle collision events per femtobarn. 1 inverse femtobarn corresponds to approximately 100 trillion ($10^{12}$) proton-proton collisions.

In 2015, the LHC collider at CERN delivered over 4 inverse femtobarns worth of proton-proton collisions to the experiments.

LHC is expected to deliver 25 inverse femtobarns in the next 6 months.

In the future, LHC will be capable of delivering 300 inverse femtobarns per year.