"Less likely" simply means that there is a lower probability of something happening than of something else happening.
"I would like to be President of the USA but it is unlikely"
"I would like to be President of the USA or Queen of England. Of the two, it is less likely that I will be the Queen." (Admittedly, in this example, both desires are unlikely!!)
"You are unlikely to find a dress made of metal."
"You might find a dress made of wool but you are less likely to find one made of metal."
Student or Learner