A person who commences a legal action is making an allegation.
To prevail in that legal action he must prove his allegation. We say that he has the burden of proof.
If at the conclusion of his part of the case he has failed so badly in meeting the burden of proof that he has not made a prima facie case, one that on the face of it has some legal merit, it can be dismissed without the defendant offering evidence.
If the case is a prima facie case, at least barely legally sufficient, the defendant has the option of not opposing it and hoping the jury agrees it has not been proven to the required burden of proof, or the defendant can go forward with his evidence to further weaken the case against him.
Even if the plaintiff has made a prima facie case (maybe it happened) he still may not have met the full burden of proof for the case. There are several standards.
1. Beyond a Reasonable Doubt. This is the highest standard to be met and it is used in all criminal cases. This standard is also used for the civil (not criminal) commitment of a sexual predator because there is no recognized treatment [or illness for that matter] and the commitment is essentially indefinite.
2. Clear and Convincing. This standard is less exacting than the 'beyond a reasonable doubt' standard, but it is nonetheless a high standard. It is typically used in civil commitment cases [danger to himself or others] but can be used in other cases as well. In Cruzan the U.S. Supreme Court upheld a 'clear and convincing' standard to be met for evidence that a person in a persistent vegetative state would want to have a ventilator or artificial nutrition and hydration stopped.
3. Preponderance of the Evidence. This is the basic standard of proof used in most civil cases, including medical malpractice cases and most other civil suits. A preponderance of the evidence is sometimes said to be 'more likely than not' or '51% chance' that the events complained of occurred. Even though this is a fairly low standard of proof, it is still higher than a prima facie case since 'more likely than not' is a bit more demanding than 'maybe it happened'.