Wikipedia has a good article explaining what High-bandwidth Digital Content Protection (HDCP) is. For the most part, when things are working correctly, you shouldn't need to worry about what HDCP is and everything will seem to just work. There are some cases were HDCP can cause problems (audio stops working from HDMI connected AV receiver or the TV just goes black when trying to play certain videos) in which case Roku support might ask you what version of HDCP is stated on the Roku and what equipment the Roku is connected to.
The short answer is HDCP is supposed to make sure you can't record video content that needs to be "protected." There are companies like Black Magic Design make devices to record from HDMI on a computer. This is useful if you have a camcorder that outputs video you created yourself via HDMI. However, several companies such a Disney aren't happy about the idea of someone streaming a movie like Black Panther from a Roku and then recording it. As such, some Roku apps will confirm HDCP status before playing that content. Since Black Magic Design can't support HDCP (it would violate the HDCP licensing terms), the HDCP check during playback to the HDMI device going into the computer will fail and playback/recording the movie won't be possible.
The update to support HDCP 1.4 is helpful in continuing to legitimize Roku as a trustworthy streaming device for playback of high valued movies as problems have been discovered with previous versions of HDCP. Complete details on what exactly those problems were or any other discussion on defeating HDCP is not allowed on this forum. The wikipedia article gives a summary on some of the known issues with HDCP's previous versions without going to specifics (and as such should hopefully not be considered to violation of the Roku forum rules or the DMCA).
As a side note, if you are looking to buy a new 4K resolution TV or a new audio receiver, it is probably best to look for one that indicates in the specifications that it can support HDCP v2.3.
"John Randwood" wrote: SO you cant turn HDCP off? on Roku
You can't turn HDCP off on any device that uses it. And if you did, you most likely wouldn't be able to stream from many sites, or even get your display to sync. As far as updating the version of HDCP your device uses, it's doubtful. There's a hardware component to HDCP that may or may not be present in your device.
Dan Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.