I know what a group being open means in the later parts of a group draw, normally teams of the same country cannot play in the same group. But I find it very peculiar on the first pot, in other words the first teams to be placed to groups some groups are not "open" to a club. Does that have anything to do with the game schedule Tue/Wed and TV viewership? So that for instance Real Madrid and Barcelona do not play the same day? Or is it a completely different story?
Teams from the same nation are spread across the two halves of the draw. So if there are 4 English teams, 2 can go in groups A to D and 2 in groups E to H. As such it can cause limitations on other teams.
There are also other limitations, such as the two Manchester clubs must be in the opposite side of the draw to help confirm that they don't play on the same night, this is due to policing laws. It can all get a bit confusing but as a general rule, the first 2 sentences will be enough.
This is due to seeding - the top seeds all avoid each other in the first round, in a similar way to other sports such as Tennis.
In your example, Real Madrid & Barcelona are both top seeds so cannot meet in the group stages.
It is basically to do with TV rights and the schedule around that. I can't remember exactly the days (I will edit later) but to use an example:
In England the rights to the group stages are bought on different days. ITV have the rights to Tuesday games (for English teams) whilst Sky have the rights to all Wednesday games. Bearing that in mind, imagine that all English teams are top seeds, now imagine, that Group A and Group B both have a first pick of an English team. The remaining two English clubs are now closed to Group C and Group D because Groups A-D will play on one day and Groups E-H on the other (swapping each game week). This is what is meant by the pot being open or closed. It would be unfair if you had brought the broadcasting rights to Tuesday's games and there were no English clubs playing on the Tuesday.