What is utf_8 in HTML5

Answer 1

It's an encodng (not just in HTML), which is a way of translating numbers that the computer understands to text that a human understands.

Not too long ago americans used ASCII to display latin characters, japanese people used JIS for japanese text and indians didn't really have anything; there was ISCII for devanagari script (and others) but it was never used very much.
If you sent an ASCII encoded text to a person in japan and they opened it without knowing it was ASCII, to them it looked like a cat walked over a japanese keyboard (google "mojibake", you may have seen it before)

Unicode (utf-8) is a global standard that works everywhere so that's great. On the web we use it pretty much exclusively.

Answer 2

it is the character set. in utf8 there are lots more characters than in ascii, like chinese characters.