Prorogation is the formal end of a session. This is not the same as dissolution, which is the formal end of a Parliament.
Motions (including early day motions) lapse. Questions which have not been answered fall and nothing more happens with them. No motions or questions can be tabled during prorogation. No committees meet.
Bills that have not completed their passage through both Houses of Parliament fall. But if the House has already agreed a carry-over order, a bill may continue in the new session.
Before prorogation takes place, Royal Assent is signified to any remaining bills that have been passed during the session.
Prorogation takes the form of an announcement, on behalf of the King, read in the House of Lords. The Speaker of the House of Commons and MPs attend the Lords Chamber to listen to the speech. The same announcement is then read out by the Speaker in the Commons Chamber. Following this, both Houses are officially prorogued and most parliamentary business is suspended.
Prorogation is followed by the State Opening of Parliament and the King’s Speech a week or so afterwards to begin a new session.
Parliament is also usually prorogued before it’s dissolved when a general election occurs.
You can watch a video of prorogation: