If the House of Lords changes a bill that started life in the House of Commons, it’s returned to the Commons with a list of the changes made. These changes are known as Lords Amendments.
The two Houses have to reconcile their differences and agree a text before the bill can receive Royal Assent. This means that the Commons has to decide whether to agree or disagree with the changes the Lords made to the bill (or vice versa for a bill that started in the Lords). The bill is sent back and forth between the two Houses as they try to resolve any disagreements. This is known as ping-pong.
When MPs consider Lords amendments, the Speaker must examine the motions relating to them to see whether they need to be certified under the English Votes for English Laws process.
Under certain circumstances a bill can be passed without the agreement of the Lords. The Parliament Acts allow the Commons to send a bill for Royal Assent even if the Lords haven’t passed it, provided that certain conditions are met and not less than 13 months have passed since the bill initially received its second reading in the Commons. In practice, it’s rare for the Parliament Acts to be used.