This is a popular sentence on the Internet. Meaning (or meaning) is subtle. For example, the question you ask can be described as your cleverness, and so can my answer. Generally speaking, it means that there is a bright spot somewhere (that is, XX), which is often not positive.

For example, "the first floor is lit up"-this often means that the first floor has replied with interesting or meaningful content. For example, in this case, the first floor replied "Laugh at the dog's head on the first floor", but he was the first floor himself, so it lit up.

For example, someone made a joke, but the joke was very low, so someone replied that the landlord was on. For example, the joke: "The polar bear has pulled out all his hair and said it's so cold", and the joke is low, which can be described as a joke.

For another example, a netizen said, "When the British invented badminton, they didn't expect a country like China." The netizen replied: "When the ancients invented cuju, they didn't expect that there would be a men's football team." This reply is also bright.

For another example, I think others will make it clear when they see this solution.