Refers to a painful, unpleasant, or regrettable experience that leaves a lasting negative impression, similar to a bitter taste.
He talked about a bitter experience from his youth.
That failure became a bitter experience for me.
経験 means 'experience' in general; 苦い経験 specifically denotes a bitter or painful experience.
Compound of 苦い (bitter) and 経験 (experience). The metaphorical use of 'bitter' for painful experiences is common across languages.