"Elephant in the room" is an idiom. It is used to refer to a big and obvious issue that everyone is intensely aware of but noone is acknowledging. The issue becomes like an elephant in the room, crowding out all the space, blocking sight lines, making it impossible to move freely. Meanwhile, the people in the room drink tea and chat to each other without mentioning the elephant at all.
Elephants in the room are often emotionally charged. Mentioning the elephant could trigger anger, embarrassment, or shame. Avoiding mention of the elephant can avoid arguments, or avoid people having to grapple with a difficult topic. The elephant may also be ignored because it's not polite to mention a social taboo.
An elephant can be quite small, something as simple as someone with their fly undone in a social setting. If you tell the person, it saves them from further embarassment. It also infers that you noticed their crotch, which is not something most people want to admit, so sometimes you just don't mention it at all and hope the person notices on their own.
Personally, I don't mention politics to my parents, because we favour different political parties and the ensuing debate goes nowhere. On a political issue, I change the topic instead of engaging with them. I treat politics like an elephant in the room when visiting my parents.
Fictional examples of other elephants in the room:
1. Two bitterly divorced parents are both attending their son's wedding. They make terse polite conversation with each other and absolutely avoid mentioning the divorce or anything to do with it at all. Neither wants to ruin the wedding for their son by re-starting the old argument yet again in public. The divorce is the elephant in the room.
2. Tom's wife died, and he refuses to talk about it. He's started drinking heavily and loses his temper if someone mentions the drinking or the death, but otherwise seems ok. After a few outbursts from Tom, his friends avoid talking about the death or the drinking. Conversation continues as per normal on other topics, and everybody pretends Tom is completely fine because it's easier that way.
3. The family is extremely religious. At Thanksgiving dinner, there is one less place setting at the table this year. Their daughter came out of the closet a few months ago and was disowned completely. During dinner, a nephew starts to say something about it and is quickly hushed by his parents. Noone else mentions the elephant in the room at all.