This summary is not the rule. The rule text controls.
If someone sues you back with a counterclaim after you filed your own lawsuit, you become a defendant on that counterclaim. This rule says that in that situation, you have the same right defendants normally have to bring a third party into the case.
In plain terms, if you started the lawsuit but now find yourself facing a counterclaim, you can pull in another person or company who might be responsible for the claim against you. This works the same way a regular defendant would use this process. The idea is to let all related claims get sorted out in one case instead of forcing separate lawsuits.
So if you're the original plaintiff but get hit with a counterclaim, don't assume you're stuck defending it alone. If someone else should share the blame or cover the cost, you may be able to bring them into the case too.