Q; SDR, what's that  ?

Software Defined Radio. 

It's a radio with the hardware parts of a real radio emulated in software.

Q: Ok so what's the advantage ?

Overall the advantage is that it can decode different modes of recieving like FT8, FM, AM, Upper and lower sideband etc.

And if a new mode becomes available the software just gets updated.

No new radio needed.

Q: So whats the Web part ?

The ability to not just run it locally but over the local Lan or a phones hotspot.

So the fact it only needs to support a web browser removes the ability to host / support on multiple operating systems.

The only requirement is a supported SDR card and the server runs on a Debian based server.

also multiple users can access the same radio and tune around (provided they dont change the current profile / view)

Q: and the Open part ?

Its Open Source so if you are geeky enough, you can improve and contribute to OpenWebRX.

Q: so what is the '+' part ?

A kind person by the name of Laurvique has added extra modes of radio decoding and then repackaged the image file for easier setup.

Q: ok clever clogs, why not just install it all on my own pc ?

You can but the idea with this project is that with a Raspberry Pi, the power usage is low so it can be ran off a USB power bank thus allowing the whole device to be placed somewhere (like in the garden (out of the rain) away from any noise but still work on the local network.

It can even be placed in the loft.

Q: if its that good why isnt everyone running it ?

Because you need to be a bit of a geek to 1: be interested in it and 2: setting up is a bit lof a pain.

However, if you have a Raspberry Pi and a supported SDR card, you can just download the image here :-