Any scientists please accept my apologies now ;D
The way I understand it is that F1 seeds are hybrids ie they come from very particular crosses of parent plants that give "good" characteristics to the offspring ie the F1 seeds we buy and grow. This could be disease resistance, uniformity, earliness, etc, whatever they've been selected for... (Although I think you sometimes have to read seed catalogues descriptions with the same suspicion that you read estate agents adverts!!)
Basically what you grow should be pretty consistent, but the seed might be a bit more expensive...
However, if you try to save seeds from the F1 plants later, you don't know what gentics you're getting from the new crosses, so you can't guarantee what characteristics your new plants might have. They might be good, they might not!!
This is fine if you like what you're growing and are happy to carry on buying seed from the seed company when you run out.
But if you want to save seeds yourself (to save money, or because you can't buy all varieties of all seeds in catalogues) then this is a problem and you have to look for non F1 varieties...
