Pico means that the digit is in the number and it's in the right place.
Ferme means that the digit is in the number, but it's in a different place (therefore having a different value).
Nada means that the digit is not in the number at all.
For example, the person thinking of the number comes up with 784. They don't tell the other players. The other players start guessing. Perhaps they guess 743.
7 is pico because it's in the right place.
4 is ferme because it's in the number, but in the wrong place.
3 is nada because it's not in the number at all.
The players then use their reasoning skills to make a new guess.
I know this sounds a little complicated, but the kids know how to play. Let them teach you! It's lots of fun. :o)