You can’t be sued (for example, for defamation) or prosecuted for anything you say in the Chamber, Westminster Hall or a committee of the House. This allows you to speak up on behalf of constituents, express an opinion, or condemn corruption, malpractice or even criminal activity without fear of legal action, as long as you do so in proceedings of the House. This protection extends to written proceedings: for example, written and oral questions, motions, early day motions, and amendments tabled to bills and motions.
Anyone giving evidence to a committee of the House also has this protection, which is a safeguard for witnesses and also ensures that select committees are not obstructed in their inquiries by threats of legal action, or any other kind of threat against witnesses.
A similar protection applies to any document published by order of the House, or under its authority. This includes select committee reports and written evidence, certain reports made under statutory authority and reports published in reply to a ‘motion for an unopposed return’.
The protection extends to Hansard (the Official Report), but if you circulate an extract from Hansard (for example, a speech) on its own, the protection may be more limited. The protection for published documents is provided by the Parliamentary Papers Act 1840.
Privilege doesn’t necessarily apply to the repetition outside Parliament of things said in proceedings.