Matter is planning to launch a monthly digital magazine (a la The Magazine) with a $.99 monthly subscription. Readers will get a single in-depth article each month which can be read online, as an ebook, and thanks to this new partnership readers will also be able to read the articles in their preferred Pocket app.
TBH, I'm not sure how this matters to anyone other than Pocket and Matter; as far as the user is concerned Matter will be as accessible as any other site which charges a subscription. After all, Pocket has enabled readers to independently enter their login info into the iOS and Android apps so they could save articles for later (more info here). This deal with Matter is simply more of the same.
It's not like Matter will automatically send the new articles to your Pocket account (now that would be newsworthy).