Please correct me, more informed people, but my understanding is as follows:
When there's been a big change in government (eg. when a new PM comes into post as Boris has just done), it's normal practise for parliament to be suspended (a prorogue) to allow them to set out their priorities. Usually the suspension is for a week or so.
BJ has asked for (and the Queen has approved) a much longer suspension than is typical. That said, it covers a period that possibly could have been 'recess' anyway (during which time most parties would hold their conferences). BJ and co. maintain that their actions are 'business as usual' and in line with normal practise.
Parliament can vote to cancel 'recess' but not against this prorogue. The Queen officially approves it, but until now this has generally been a ceremonial thing, rather than a politicised move against the clear will of parliament.
There is a view held by some that we are in exceptional circumstances with regards to Brexit negotiations, and every second of parliamentary time is vital to get the best possible outcome.
The uproar about this prorogue is linked to the belief that it's a cynical move on BJ's part. By 'stealing' time from active parliament, he is disabling his opponents from developing plans / proposals which could thwart his desired Brexit plans, or put his position as PM in jepordy. It appears that a No Deal Brexit is now much more likely, which comes with its own set of risks.
Have tried to write this in as non partisan a way as possible but not sure I've managed! 😂