Frank's Son Brian and former Star Wars book writer Kevin J. Anderson teamed up to write a whole ton of other Dune Prequels, Interquels, and Sequels.
If you're a Dune fan and have the time (especially during quarantine), the books are "OK"...some series better than the others. But as abake mentioned, they are nearly NOT as well-written and detailed as Frank's novels.
-"The Butlerian Jihan" prequel trilogy is probably the best of the Herbert/Anderson novels. It outlines the origins of the Dune galaxy and a lot of the Houses...including the Harkonenn/Atreides rivalry, the "spark" and war for the destruction of the Thinking Machines, and the origins of a lot of the other entities such as the Bene Gesserit, Swordmasters of Ginaz, Mentats, Fremen, The Guild (and Navigators), etc. Some of the concepts are interesting and the origins for the different elements play out in these books.
-"Prelude to Dune" follows many of the characters leading up to the first "Dune" book. It follows Leto in his younger days, even details him meeting Jessica and why she eventually chose to bear him a son. You also follow the origin of Duncan Idaho and how he ends up with House Atreides and why he's so loyal to them. It also follows Vladimir Harkonnen and how he rises as the Baron and his various manipulations. I think it finalizes with House Corrino and how Shadam IV becomes Emperor.
-There were some other ones like "Paul of Dune" and I think some further House origin books but I didn't read those.
-The Herbert/Anderson "Dune sequels" are "Hunters of Dune" (Book 6) and "Sandworms of Dune" (Book 7). These basically read like Dune fanfics and are nowhere near the caliber of Frank's series. They take some key elements from Herbert's/Anderson's prequel series (I won't spoil what) and slap them in here. It's basically a "let's get the band back together to kill the ultimate bad guys" plot. You're better off leaving the high on Chapterhouse and ignoring these or reading what happens on wiki...the writing is probably just as good if not better on the wikipedia page anyways.