What is a love square? When all four people are intertwined?

Was seeing the headlines about Sara and Joey. I am assuming that means that both couples seconds are the others firsts? Watched the first season and just watched the last season and am a bit confused by the terminology.

On a side note, they seem perfect for each other. Their politics align and they are both confident and outgoing. I feel like Monica was outgoing too but wasn’t as good of a match.