It means the plants have never been in contact with frogs so there is little chance of transmitting disease. If they were in my viv with frogs and I sold plants to you from my viv and my frogs are infected there is a high chance that you could infect your frogs with these plants. Just another way to ease fears of infecting others frogs.
It might be a good idea to go ahead and sterilize any plants you get, just in case. I use a bleach and water solution, like Pumilo noted. All plants have survived except for riccia. Don't soak riccia in bleach. It does ok with alcohol, in my experience. Alcohol does not kill snail eggs, also in my experience
To be honest I dont worry horribly much about getting frogfree plants as long as it isnt someone tearing down their viv(I think the chances of a sick frog involved are higher)
However I do ask whether the plants are frogfree or not and notify the seller that this will not effect my decision to buy I just need to know. This determines the priority on sterilization. I may wait a bit on frogfree plants if Im busy or out of bleach. and in turn just put them in a growout tank until they root then I pull them and clean them. However if the plants are not frog free they immediately get sterilized before touching the rest of my collection. But overall I try to bleach every plant as it comes in.This reduces exposing my own collection to pathogens that could be harmful to the plants as well as ones that are harmful to the frogs
Before any viv is planted EVERY plant going into that viv is bleached with a 5% bleach solution for about 6 minutes .. (Ive found 10 to have high failure rates in the plants surviving) The rinsed extremely well and most of the time I also use a dechlorinator solution in the rinsing cycle as well.