How is hexadecimal number 0x3F converted into decimal?

I recently saw this dude William Lin use this hexadecimal 0x3F in memset() where in the place where a character variable should be passed. Does 0x change the type of 3F. I think 3F in decimal is 63. And the in the ascii table, 63 corresponds to '? ' symbol. Instead of using the question mark, why did he use the hexadecimal? Or is there some other explanation? I really need to know! Thanks!