The whole idea behind this is, that in many countries the satellites or terrestrial TV send more than one channel on the same "highway". Currently MythTV does only record one of the lanes in this highway. Other projects like VDR allow the recording of more than one lane, reducing the amount of TV cards that one needs to record simultaneously.
Let me show this on a practical example based on the Astra satellite that hovers over Europe. The transponder 107, to continue with my analogy is the highway, includes the channels (lanes) 9Live, Kabel 1 Deutschland, N24, ProSieben Deutschland and Sat.1 Deutschland. Now imagine the situation where at a set time, for example 12 pm, you have a TV series that you have set to record on ProSieben every day. Now at 12:15 pm, Kabel 1 has a nice Movie showing you would like to record. As of now, MythTV has to select one or the other based on your priorities or you have to buy a second TV card to be able to record the other show. An even worse scenario would be that at the same time a third show on Sat.1 would start… you get my point.
This is not a trivial task to program and that is why I have send my 25,- Euros to help the cause. Just imagine how many TV cards you will save by doing this.
For further detailed technical description of this problem check out the MythTV Wiki.
I just wanted to bring to everyones attention the fact that a few developers have pledged to the development of the code necessary for recording multiple channels from one multiplex. Check out this page to contribute and see how far along we are. This is a great way to improve MythTV for those of us who do not have coding skills or time.
UPDATE: The full 6.000,- Euro have been collected! Thanks everyone, we might soon have this feature.