What does pleading the fifth actually mean?

The Fifth Amendment protects against being compelled to be a witness against yourself, so pleading the fifth means declining to answer a question on the grounds that a truthful answer could tend to incriminate you. It applies in criminal trials, grand juries, congressional hearings, depositions, and police questioning. A criminal defendant can decline to testify at all, and the protection covers testimonial evidence rather than physical evidence, which is why courts have allowed compelled fingerprints, DNA samples, and blood draws under proper procedures while protecting compelled statements.

Two nuances matter. In a criminal case, the prosecution may not comment on a defendant's silence and jurors are instructed they cannot infer guilt from it, a rule from Griffin v. California, which is why the popular reading of invoking the fifth as a confession is legally wrong. In civil cases the rule differs, and a fact-finder may be permitted to draw an adverse inference from a party's refusal to answer, which is why the same invocation carries very different weight depending on the proceeding. The privilege can also be lost: it can be waived by testifying selectively about a subject, and prosecutors can neutralize it by granting immunity, after which a witness can be compelled to testify because the answers can no longer incriminate them. Anyone actually facing this should be making the decision with counsel rather than from general principles.