SVN is a repository, but you do not work directly on the data within it.
First, you should branch the files you are working on. The action of branching tells the SVN repo that the original version of the files should be copied locally and disconnected from the repo, as a brand new version of those files. This is critical when you have multiple users working on the same repo, to avoid conflicts and problems in general.
Then you can do your work and modifications on your local copy.
Once you are done, you can commit your branch, have it reviewed, tested, and in the end merged back into the main trunk. If some other mod was done to the same files you committed while your mods were being reviewed, you will encounter a conflict. At that point you need to manually review the mods of each versions, and decide what to keep from each.
Think of your SVN repo like a snapshots collection. It is NOT a file-sharing server.
