When you are arrested or questioned about an offence, you must be cautioned. The standard caution is:
"You do not have to say anything. But it may harm your defence if you do not mention when questioned something which you later rely on in court. Anything you do say may be given in evidence."
Breaking that down: you have a right to silence — you do not have to answer questions. But if you stay silent about something and then rely on it in your defence later, a court may be allowed to draw a conclusion from the fact that you did not mention it earlier. And anything you do choose to say can be used as evidence.
This is why it is usually sensible to get free legal advice before answering questions. Your right to free, independent legal advice in custody is separate and is not affected by the caution.
This is a plain-English summary. For the exact rules, read the official PACE Codes.