In Mandarin Chinese, 日 (Pinyin: rì 🔊) means “Sun”, “daytime” (time between sunrise and sunset), and, by extension, “day” (i.e. twenty-four hours). It is also short for 日本 (Rìběn), “the Sun’s origin”, that is, Japan. Combining 日 and 出, we get 日出 (rìchū 🔊), “sunrise”.
Just like 入 (rù), 日 can be used in a sense “to have sex”. It is not clear how this meaning originated. It could be that some of the historical forms of the character were suggestive of this vital activity. Note the similarity with the Western solar symbol, ☉, which is also the alchemical symbol for gold.