"they" are the people who are
accountable for anything that you can't remember the original source for. If you know an interesting
proverb or sound peice of advice, you use the term of "they" to attach it to, so when things go horribly wrong from said advice, the imaginary "they" group can take the responsibility off your shoulders. Also a way to seem smart by remembering good advice, without having any
hard facts to back it up.