Bad Taste In One’s Mouth

What does the idiom “Bad Taste In One’s Mouth” mean?

Definition:

Unease, a feeling that something unspecified is wrong in a situation

For example:

“The agreement is OK, I guess, but it leaves a bad taste in my mouth—I’m afraid that it will bring us real problems in the future”

Notes:

This is generally used with the verb “leave”—”leave a bad taste in someone’s mouth.”