When you learn real English you will hear many English language idioms (both American idioms and English idioms).
In this video Kristin talks about the American English idiom “The Grass Is Always Greener (On The Other Side)”.
If someone says “the grass is always greener (on the other side)”, then that person is telling you that even though another situation might seem better than yours it often is not any better.
Idioms are commonly used in real English conversation. So if you want your English speaking to improve, then learn English language idioms.