Apple used to provide the SVN command line client with XCode (it still might) but it never provided the JavaHL library or any of the language bindings, so it is best to get Subversion from a package manager.

I use and recommend Homebrew. If you are doing software development on OSX, having this installed will make it easy for you to get a lot of tools you will eventually need. Once you have Homebrew installed, installing JavaHL is pretty simple:

brew install subversion

After the installation, you need to run these commands which put a link to the JavaHL library into the folder that the JVM looks for JNI libraries. This causes it to automatically be seen by the JVM when you run Eclipse.

// Make sure this folder exists (it should already be there)
sudo mkdir -p /Library/Java/Extensions
// Adjust the path for your Homebrew installation
sudo ln -s /opt/homebrew/lib/libsvnjavahl-1.dylib /Library/Java/Extensions/libsvnjavahl-1.dylib

If you use MacPorts or another package manager, there would be similar ways to do this.