Members of the House of Commons and the House of Lords take an oath of allegiance to the Crown. This is called swearing in. Members can either swear in using a religious text or take a non-religious, solemn affirmation. They do this before taking their seats in Parliament. This can happen after a general election or by-election.
As in this case, swearing in also happens after the death of the Monarch: for members of the House of Lords this is an obligation but for MPs it is optional and not doing so has no effect on them taking their seat or participating in business.