Q:What does (P) means after a font in the list?


(P) means “Proportional”, which means fonts with proportional space, while other fonts are “monospace”.

Proportional fonts take no effect on Chinese characters. It is effective on Roman alphabet, symbols, numbers, etc. which require fine-tuning of width to facilitate reading. As what the word suggests, “monospace” characters are characters of equal width.
The examples of “Medium Hei” below show the difference between “Medium Hei (P)” proportional and “Medium Hei” monospace characters: