Something about just about every person that shows that they are part of the human race and they take what is shown to them, and accept it as the truth no matter what, and that they need to do certain things that are in the "mainstream culture".
A person who is easily duped or persuaded. An impressionable person most often tends to be younger, an unwise to the ways of the world. Inpressionable people are often incapable of critical thought.
Nick: I've seen his YouTube channel twice.
Colin: I'm basically his biggest fan /s.
Nick: Really?
Colin: No, I'm just trying to impressionate you.
<Cue hilarity>