The pleadings stage is the initial stage of any litigation. The Plaintiff (or the person bringing the action) will file a summons and a complaint. The summons will indicate that the Defendant(s) have either 20 or 45 days (depending on the allegations made) to answer the complaint.
The complaint will contain all of the allegations that the Plaintiff says the defendant(s) did or did not do. For example you may see a complaint containing several counts, one may be breach of contract and another may be Fraud or Misrepresentation although they may all be referring to the same facts.
As a defendant you have the obligation to file an answer either 20 or 45 days (depending on what is plead) of being served with the summons and complaint. If you do not answer, a default judgment may be entered against you. (Plaintiff automatically wins). If you are a corporation or an LLC, you have to get a lawyer to enter an appearance and file an answer for you.
Your answer to the complaint is not the time for you to make your case as to why you are right and they are wrong, but rather is simply a format for admitting or denying each allegation made by the Plaintiff. Additionally the defendant(s) will take this opportunity to indicate any affirmative defenses and put forward any counterclaims or cross claims that they may have.
Once all of the pleadings have been filed, and any necessary amendments have been made, the discovery phase of litigation begins.