Tybalt's use of 'heartless hinds' is a metaphor. He is referring to the servants of the Capulets and Montagues who are fighting each other. Benvolio has just intervened in the quarrel. Tybalt taunts him for drawing his sword in the presence of what he sees as mere, lowly servants.

This image used by Tybalt here to describe the servants is a very disparaging one, comparing the servants to female deer (the word 'hind' refers specifically to the female of the species). Tybalt thus characterizes the servants as being weak and womanly. His use of the word 'heartless' compounds this effect as he is punning on the word 'hartless', a hart being a male deer. With this phrase Tybalt thus disdains the servants as being feeble, unmanly and also wicked: 'heartless'. At the same time, he is mocking Benvolio for being embroiled in a quarrel between what he sees as weak and feeble creatures. He goes on: 'Turn thee, Benvolio! look upon thy death.' He is provoking Benvolio to a fight here by jeering at him, by implying that he is not a real man until he turns and fights with a worthy opponent (i.e. Tybalt himself). 

In the very beginning of Romeo and Juliet, Tybalt jests with an absolutely glorious pun!  Of course, the feud between the Montagues and the Capulets is in full fire, as members of both families fight in the streets.  Suddenly, Tybalt appears and spouts his first line:

What, art thou drawn among these heartless hinds? / Turn thee, Benvolio! look upon thy death. (1.1.65-66)

In Tybalt's very first sentence, he utters both a threat, an insult, and a pun!  (LOVE it!)  Generally, Tybalt is trying to provoke Benvolio into fighting by calling the Montagues "heartless hinds."  On the surface, "heartless hinds" simply means "timid servants" (an insult in itself); however, if one looks closer and discovers the etymology of the word "hart" and "hind," one discovers and even deeper grating comment in Tybalt's pun.  The word "heart" is a play on the word "hart" which means "a male deer."  The word "hind" in addition to meaning "servant" can also mean "a female deer."  Therefore, Tybalt is basically throwing the ultimate insult at the Montagues in that Tybalt is insulting their manhood:  "What, art thou drawn among these man-less women!?!"  Whew!  Does Tybalt know how to throw them, or what!?!