+1
"+1" is a two-character forum reply used to signal agreement with the post directly above it. The phrase was formally documented on Urban Dictionary in April 2004, and it later became core jargon in the Know Your Meme comment sections, though its use faded across the 2010s as upvote and "like" buttons took over the same role.
Overview
"+1" (read as "plus one") is a short forum reply used to signal agreement with the comment directly above it1. Instead of typing out a full response, users drop the two-character shorthand to add weight to another person's opinion or to co-sign a suggestion someone has floated for community consideration1.
The reply carries a positive connotation but does not always mean full endorsement of an idea. Sometimes a "+1" is just a call for the specific action the commenter above suggested, not a blanket agreement with their broader argument1.