When you are suing the landlord, they usually file a motion to strike portions of your Complaint [=throw out part of your case] by a Motion to Strike. Usually, it's to get rid of punitive damages [=extra penalty for bullying you] or attorney fees [because you are officially without a lawyer], but they attack other things. You need to oppose that motion or have your case whittled away.
This is that Opposition. We don't know what the basis will be, so there may be additional charges if it is other than the normal motion we are prepared for. It is always filed with a Demurrer to your Complaint,which also requires an Opposition to avoid more whittling away at your case. The demurrer opposition is a separate charge [Item #377] and process.