When you check out a project code base from a svn repository, each downloaded directory (from top to the deepest) contains a .svn hidden directory that keeps svn’s necessary metadata.
If you want to remove them all at once, here’s one way to do it:
~/project_dir $> find -name .svn -print0 | xargs -0 rm -rf
find -name .svn searches the current directory hierarchy for files named .svn.
-print0 ensures each filename ended with a null character. This allows filenames that contain newlines/whitespaces to be interpreted correctly by programs that consume the output.
xargs -0 rm -rf receives the output and passes it to rm for file removal. Remember to use -0 option when the input items are terminated by a null character.