Often beleived to be 'Yes' in Japanese, this is not quite accurate. It is better to think of Hai as 'that is right', and its negative counterpart, Iie, as 'that is wrong'. Why? The Japanese think of them this way. When answering a negative question in english, you do something like this:

"The air conditioner is not working, is it?"
'NO, it is not.'
Or
"The air conditioner is not working, is it?"
'YES, it is (working).'

the Japanese would answer this way:
"The air conditioner is not working, is it?"
'YES, it is not.'
Or
"The air conditioner is not working, is it?"
'NO, it is (working).'

Seem confusing? Not when you realize that Hai is 'that is right', and Iie is 'that is wrong':

"The air conditioner is not working, is it?"
'That is RIGHT, it is not.'
Or
"The air conditioner is not working, is it?"
'That is WRONG, it is (working).'