It’s all explained here:
https://commonslibrary.parliament.uk/brexit/negotiations/the-benn-burt-bill-another-article-50-extension/
In precis, by 19/10 the PM must seek and secure the agreement of the Commons to either leave with no deal or agree a deal. When he does not he must ask the EU for an extension to 31/1/20. If any time after 19/10 the Commons either agrees to leave without a deal or it agrees a deal the PM can withdraw or modify his request.
If the EU agrees to an extension to 31/1/20 the PM must inform the EY that we accept it (this was not explicit in the Cooper/Letwin Bill).
If the EU offers an extension to a date other than 31/1/20 the PM can either agree that date or come back to the HoC and ask them to approve it. But then:
The Bill also gives Parliament, and the House of Commons in particular, an ongoing role in scrutinising progress towards the securing of a deal between the UK and the EU. The Government must publish a report on 30 November explaining what progress it has made in this regard. MPs would then, by 5 December, be asked to ‘approve’ that report.
If MPs were not to approve that report or were to amend the report’s approval motion the Government would then have to set out a further report explaining what it proposes to do in the negotiations. This second report would have to be published by 10 January 2020. This reporting and approval requirement then repeats every 28 days until either a deal has been reached with the EU or the Commons resolves that the requirement should cease.
Quite what the Commons hopes to achieve with this latter clause is a little unclear. It is obvious that the EU will not change its only deal on offer (with “No Deal” being explicitly ruled out) so all that producing a blank report every 28 days will secure is a bit of ongoing employment for a few scribes.
The Commons needs to accept (and declare to the public) that its aim is no Brexit at all. All this fannying around with reports on nothing is simply smoke and mirrors. A general Election is needed to ask the country who they want to run this process because at present it is no process at all.