http://en.wikipedia.org/wiki/F1_hybrid
Some cool info, but kinda technical.
F1s are the first generation hybrids, a cross of two plants with unique characteristics. Like a short, sweet indica x tall, skunky sativa resulting in a F1 cross that is short and skunky. ( just for example, could be the other way too )
some useful quotes from wikipedia :
"Once the characteristics of the cross are known, repeating this cross will yield exactly the same result."
"Both inbreeding and crossing the lines [typically by hand] requires a lot of work, which translates into a much higher seed cost"
"The main advantage of F1 hybrids in agriculture is also their drawback. When F1 cultivars are used for the breeding of a new generation, their offspring (F2 generation) will vary greatly from one another.."