Person (in this case a guy) took his girlfriend/wife for granted.
After she is gone he can now appreciate what he ONCE had.
"Only know you love her when you let her go"
Well you only need the light when it's burning low
“You only need something when you’re running low of it”
You only miss the sun because the snow makes everything cold and dull
"Only miss the sun when it starts to snow"
You once had it all and after you don’t have anything you can then see what was one there
"Only know you've been high when you're feeling low"
Generally, most people enjoy travelling or driving around but once you’re sick of it or start hating it, one obviously starts missing home.
"Only hate the road when you're missing home"
These lyrics fit my 'current relationship' so well. So touching.