A ‘free bid’ is one that you make after your right hand opponent has overcalled your partner’s bid. The overcall ensures that your partner will have another chance to bid and therefore you have the option of passing. A ‘free bid’ in a suit will normally show a better than minimum suit or extra values.
Since West could pass over 2♦ knowing that East will have another call, 2♥ shows either a six-card heart suit or perhaps a very good five-card suit eg with 100 for honours.