This is more like a "Maybe" than a straight out "No". If you are interested but you don't want to say 'yes' just yet, use this. If the person is sincere about the request, he/she will be more than happy to wait a short while. Specify a date / time-range (say, in 1-2 weeks) where the person can expect a reply.
这句话更像是表示一种可能，而非直截了当地说“不”。如果你对对方所提出的事情有兴趣，但又不想马上答应，那就用这句话吧。如果对方的请求是真诚的，那么他/她反而更愿意等上一小会儿。记得告诉对方一个明确的日期或时间范围（比如，一到两周），让别人知道何时能收到答复。