いい is an adjective that means ‘good’ in Japanese. It can be a little bit difficult to use at first, as it conjugates differently to other い-Adjectives.

• これはいい映画（えいが）だ。
  This is a good movie.
• これは良（よ）くない。
  This is not good.
• 昨日（きのう）の天気（てんき）はよかった。
  Yesterday’s weather was good.
• 昨日（きのう）の天気（てんき）はよくなかった。
  Yesterday’s weather was not good.

In these sentences, we can see that the first い in いい changes to よ, depending on the form. This is unique to いい, and does not happen with other adjectives.
いい can sometimes be seen as 良（よ）い, in these cases, the kanji is likely to be read as よい, and not いい. There is technically no difference between these two meanings, but よい does sound more formal/old-fashioned.

• これはいい車（くるま）だ。
  This is a good car.
• これも良（よ）い。
  This is also splendid. (‘Splendid’ is similar to the nuance that よい gives いい)