When partner opens and RHO overcalls, any bid by you is considered a "free" bid because you do not have to bid with a 6-10 HCP hand in order to keep the bidding open for opener. A free bid on the 2 level normally promises 11+ HCPs.
Therefore, a 2-level "Negative Free Bid" shows 5-10 HCPs (a "negative" response) and a good suit.
With any hand with 11+ HCPs, you double first, then make your 2-level (or higher) bid.
This convention allows a player with a weak hand to show his suit at a lower level than if he passes initially and bids his suit later.
A Negative Free Bid also normally denies support for opener's suit, since with a weak hand and support, you should always raise rather bidding a new suit.