I think the Subversion documentation is a good place to start. I found that Wikipedia doesn't really help, since it only covers a 'fundamental' point of view.
In the Subversion Book that alex mentioned, I'd especially recommend Chapter 1, although that might be on a level that's too low if you already have some experience with Svn. Chapter 4 covers branching and merging in detail, but it's quite technical.
What helped me a lot is the Daily Use Guide in the TortoiseSVN documentation; it covers the most important operations in a tutorial style.
I guess the most important things you need to grasp are branching, merging and tagging. Understanding those takes time and practice, so I'd strongly recommend a small pet project in a local repository, so you can experiment. I think it's important to realize is that the whole system is diff-based: a merge is nothing more than automatically applying the changes that have been made in one branch to the code in another branch, instead of correcting the code yourself. Stuff like conflicts (which took me quite a lot of time to understand) are just consequences of that.
But of course, I'm still learning as well :)
