Frans' contribution to the conversation about maintainable code deserve its own post, but I would like to mention this part in particular:
you will not [understand the code]. Not now, not ever. And not only you, but everyone out there who writes code, thus that includes me as well, will not be able to read code and understand it immediately.
You know what? That is not limited to bad code. I had hard time grokking good code bases, simply because of their size and complexity (NHibernate and Windsor comes to mind). Other code bases are as large, but they have easier approachability, probably because they are dealing with less complex domain an tend to have a wide coverage rather than deep (MonoRail comes to mind).
A while ago I was involved with an effort to migrate an ancient system to SQL Server 2005. The system compromised of over 100,000 lines of code, spread over some thousands of files scattered randomly in a case sensitive file system (you can guess why this is significant). The code base was about 85% SQL and 15% bash shell scripts. The database in question was a core system and contained slightly over 4,000 tables. One of the core tables was called tmp1_PlcyDma and was used to do business critical processing. That code base took data driven code generation to a level I have never seen before. I gave up trying to track down 7(!) levels of code->generating code->execute code->generating code->rinse->repeat
To say that the code base was bad is quite an understatement. To mention that the only place where I could run the code was by using a telnet console into a test environment that was not identical to production is only the start. I could mention no debugging, runtime of ~5 hours, test time of ~3 hours, etc. The code grew organically over a ten years period and you could track the developer progress from merely annoying to criminally insane (he invented his own group by construct, using triple nested cursors and syntax so obscure that even the DBA that worked with the system for the last 5 years had no idea what was going on).
Perhaps the thing that I remember most from this project that we had a bug that kept two people hunting after it for three weeks. The issue was a missing ';'. Oh, and the criteria for success in this project was successful migration, with bug-per-bug compatibility, and no one really knew what it did, including the authors(!).
But, you know what, after a month or so of looking at the code, it got to the point where I could look at something like pc_cl_mn.sql and know that it would contains the monthly policy calculation, and that this piece of code was doing joins manually via cursors again, that plcy_tr_tmp.sql was the "indexes priming" script, etc, etc.
The code was still a horror, but once you understood that the authors of this code had a... "special" way of looking at databases, you got to the point where you could get the point of the code in an hour instead of a day, and then move that to a saner approach.
So, what does this horrifying story has to do with Frans' point above?
The premise that you can read and understand code immediately is highly dependant on what you are familiar with. I know of no one that can just sit in front of an unfamiliar code base and start producing value within the first ten minutes. But on a good code base, you should be able to be able to start producing value very quickly.