According to BBC in Government asks Queen to suspend Parliament:
The government has asked the Queen to suspend Parliament just days after MPs return to work in September - and only a few weeks before the Brexit deadline.
A bit later it exposes the rationale behind this:
(...) it was thought Labour leader Jeremy Corbyn would call for an emergency debate in the Commons next week, giving MPs a chance to lay down legislation designed to ultimately stop a no-deal exit.
But if Parliament is suspended on 10 September, as is suggested, it will only give opponents a few days next week to push for their changes.
And in here they explain what it means to have a prorogued Parliament:
When Parliament is prorogued, no debates and votes are held - and most laws that haven't completed their passage through Parliament die a death.
This happens every year, so it would be theoretically normal, but in this specific case it is blocking the Parliament for as much as one month, when there are just two months to go for October 31 2019 (the date when the Brexit is supposed to happen no matter if there is no deal.)
To my understanding, this would go against one of the key points of democracy, which is Separation of powers: by suspending the Parlament, the executive would block the legislature.
What I miss here is what should be the role of the Queen here: can she blindly accept the request from the Prime Minister? If she did, what could the legislature do to force the Parliament to be reopened? Can the judiciary intervene here?