Page FISA verification material "went missing", had to be recreated
The "Woods file" is an element of the FISA process that records the results of verifying the claims within a FISA. It is intended to be documented proof that the FBI has investigated the claims they are making to the FISC, and that the claims are verified as true. This reporting indicates that the original Woods file for the FBI's verification efforts disappeared (while the FISA was still active!) and had to be recreated for the last renewal of the application by Mueller's team.
This does two things. First, it protects the main FBI personnel who worked on the FISA, because we all know that if there was a Woods file, the only thing it would possibly show is "unverified" next to just about everything. If that's gone missing, though, it's that much harder to prosecute people for those first three FISAs. In effect, the Woods file is evidence of misconduct surrounding those first three FISAs.
As for the last one -- the need to recreate the Woods file would give the FBI on Mueller's witch hunt a chance to do everything over. They knew they wouldn't, couldn't, verify the claims in the FISA based on what they had before. But maybe they could find new information in their records or the records of the campaign and transition team (which, remember, they acquired very early without normal process). They could put that into the recreated Woods file and maybe no one would look too closely at the dates.
It also provides the FBI a do-over about their investigation. They can talk to everybody again if they want to, and ask more or different questions. It's basically an opportunity to cover up any mistakes.
How successful they were can probably be measured by the fact that they got just one renewal from their efforts and then gave up.
But if the file "went missing"... well, that's destruction of evidence and obstruction of justice in light of the current investigation, isn't it?
It's amazing that this is the first we are hearing of it.