Femto is like "milli" or "micro", but equal to 10 to the -15th
or 0.000000000000001
A "barn" is a measure of area (10 to the -28 square meters)
or about the size of the uranium nucleus
(femtobarn = code word from WWII days of developing the atomic bomb)
And from my old math books I finally get use that famous phase:
It naturally follows that: 
The inverse femtobarn is how many particle collision events per femtobarn.
Oh, and here is another phase.
The derivation is left to the reader. 
Oh, oh. Here is something that tickled me...
The Higgs particle was named the "God particle" by Peter Higgs, who said,
"No one around him would allow it to be called "that God damn particle".
.