Evidence

Evidence is the raw material of a trial. Both sides offer evidence; the judge decides what the jury (or the judge alone, in a bench trial) gets to see and hear; the jury weighs the evidence and decides the facts.

The rules of evidence are technical. Federal Rules of Evidence (and each state's equivalent) cover what counts as relevant, what kinds of statements are too unreliable to admit (hearsay), how documents have to be authenticated, when expert opinions can come in, and dozens of other issues. Pro se litigants who don't know these rules can lose at trial because their best evidence got excluded.