# An evil and adulterous generation seeks for a sign ... given to it

Jesus is speaking about the people he is talking to. Alternate translation: “You are an evil and adulterous generation who demands signs from me ... given to you”

# adulterous generation

Here “adulterous” describes people who are not faithful to God. Alternate translation: “unfaithful generation” or “godless generation”

# no sign will be given to it except the sign of Jonah

"The only sign that will be given to it is the sign of Jonah" 

# the sign of Jonah

"what happened to Jonah" or "the miracle that God did for Jonah"