When a rights issue takes place an investor is usually offered the chance to buy new shares in a firm at a discount to the current price.
So for example in a 1 for 4 rights issue where the current share price is £2 each new share might be offered for £1. This implies an ex-rights price of £9/5 or £1.60. An investor not wanting to buy any more shares can sell his rights on nil paid. Here the implied value of those rights is about 60p i.e. the gap between the subscription price and the ex-rights price.