Literally 'heart-eye' — the eye of the heart, meaning one's inner mind, shrewdness, or capacity for tolerance. The 儿 suffix marks this as colloquial Beijing-style Chinese.
心眼儿 changes meaning with different adjectives: 心眼儿好 (kind-hearted), 心眼儿小 (petty/narrow-minded), 心眼儿多 (scheming/calculating), 心眼儿大 (generous/easygoing). It's a very versatile and colloquial expression.
