What are "wandering eyes" in the lyrics below?
When will I see you again?
You left with no goodbye, not a single word was said
No final kiss to seal any sins
I had no idea of the state we were in.
I know I have a fickle heart and a bitterness
And a wandering eye, and a heaviness in my head
But don't you remember, don't you remember?
The reason you loved me before,
Baby please remember me once more.

Question

What are "wandering eyes" in the lyrics below?
When will I see you again?
You left with no goodbye, not a single word was said
No final kiss to seal any sins
I had no idea of the state we were in.
I know I have a fickle heart and a bitterness
And a wandering eye, and a heaviness in my head
But don't you remember, don't you remember?
The reason you loved me before, 
Baby please remember me once more.

Answer

It means you still look at other attractive men or women while you are in relationship.

Answer

"wandering eye" means "I look at other men", "I find other men attractive". Imagine she is having a conversation with her boyfriend, but instead of looking at him, her eyes wander away from his eyes and instead she looks at the man behind him.

Answer

And the opposite of this phrase is: "I only have eyes for you." (I don't look at other men.)