It means that the person referred to is promiscuous. It is mildly derogatory, depending on the context and never a compliment.

This phrase is neither negative or positive. It's merely a statement that a person is dating many people. I've read that this term comes from British horse racing.

To play the field means: She/He is not ready to settle down with one man or woman- She /He is playing the field . He/She is romantically or sexually involved with a number of partners.

The literal meaning comes from gambling. In the U.S. dice game of "craps," the betting table layout--see http://tinyurl.com/jqx7g4b includes a standardized bet call "the field." To "play the field" means to place a bet on seven different numbers: 2, 3, 4, 9, 10, 11, 12. It is like placing seven bets at the same time. You if any those numbers is rolled. According to an idioms dictionary, in British horse racing "playing the field" meant to bet on all of the horses except the favorite. In any case, it means to make many small bets at the same time, instead of making one big bet. It's awful, but it used to be common for men to think of romance as a contest or a gambling game. A gambler who "plays the field" makes many different bets at the same time. By analogy, a gambler in the game of love who "plays the field" dates many different people at the same time.