Cambridge Dictionary says the term is mainly used when admitting something that might be embarrassing, or when someone is trying to tone down a criticism or complaint so it is less likely to offend the subject. It conveys a similar meaning as saying, "to be honest," "if I'm being honest" or "honestly" when delivering an opinion. The website uses examples including, "Ngl, I was staring at his photo for about half an hour," and "That was tough, ngl."
A beard is a person who - knowingly or unknowingly - dates or marries a gay person so that that individual can hide their sexuality. By appearing as though they are part of a heterosexual relationship, the idea was to negate any possibility of them being seen as queer.