Voting up needs 15 reputation, voting down needs 125 reputation.
If a user with enough reputation to upvote but not to downvote attempts to downvote, s/he is presented with the message:
Thanks for the feedback! Votes cast by those with less than 125 reputation but do not change the publicly displayed post score.
This sentence is not strictly correct. Upvotes do change the publicly displayed post score. The sentence should be rephrased to correctly state that it is only referring to downvotes.