There is a thing in psychology called triangulation. So person A & B have an issue, instead of working it out A talks to a 3rd person C. So A & C join against B and things get worse. So don't bring others into your conflict, work it out even if it's uncomfortable.
Usually in this case it is more venting/bad mouthing. The third person gets dragged into the conflict because the 2 do not want to confront eachother. Usually happens when parents fight and one will talk to a child about it or make remarks about the other parent.
