It's an idiom which means "In two minds about something" or basically, half-hearted, not completely focused on something.

It is in contrast with "一心一意“, which means focusing on one thing in a time. In opposite, "三心二意” means dealing with multiple things in a time. Usually it is used as a negative word, saying you are always distracted when you are dealing with your works, or saying you are not well focusing on your conversation or relationship with your girlfriend. =) Any other questions please let me know~

你在跟你女朋友吃饭时，心里却想着回家看球。。。通俗易懂，有没有！！！天才

【本意】亦作“ 三心两意 ”。一个人有三颗心，两个意念。古人认为思考是心，所以，三个心，就不能形成一致，两个意念，也就不同一处。 【比喻义】 常指不安心，不专一，同时做几件不同的事情。 形容犹豫不决或意志不坚定。[

做事不专心，不专一。